I had 2 BKs, 17 months apart and later, 3 SKs.  When DH & I got together, all 5 were close in age, 6 years from the oldest to the youngest.  They were 11 down to 5. So, my situation wasn't like yours.

But if the finances and logistics are reasonable, and it sounds like they are, I'd have another soon.  I was an only child for 8 years and wished I'd had a sibling close in age, I didn't even have cousins.  I have a brother 8 years younger, my sister was 10 years younger and anothrr brother 17 years younger.  Sadly, we are not very close, same mother but its like they are from a different planet.   That age difference is a killer.  So, when I had DS, I planned to have another so he would have that sibling.  He and DD played together, i'm sure they enriched each other's childhood.

Your SD sounds like a nice girl but the age difference means she probably won't be that close to your BKs. I'm not kidding you, the early years are really taxing but I think its worth it.

For all my complaining on Steptalk about my steps, I'm glad I had them, too.  The 5 of them had many adventures, common friends, lots of memories (although it was overwhelming).  Good luck.

I know this is out of your control but do you have a preference for gender? Not that it should matter to much in the long run but would if affect you if you had another boy? My sister wanted a girl and got a boy. Wouldn’t trade her boy for the world now but if she got pregnant again and ended up with 2 boys I think there’s a high chance she’ll spiral into a strong case of PND and gender disappointment. Hard enough to cope with in a first family situation but it’ll be more of a struggle with a step daughter involved. I admit I would’ve resented BM for having a daughter with DH if I didn’t have DD. Irrational but it is what it is. Also, has your SD ever expressed a wish for a little sister or will she be intensely jealous? Not her call at the end of the day but her reaction is something to keep in mind so your DH can nip it in the bud if needed. 

I flip flopped on having another. Two things made up my mind: 

1) I knew I wouldn't regret having another child. But I might regret not having another child.

2) I always wanted to have 2 kids. I also have 2 stepkids, but they didn't lessen my desire to have another child at all. I would feel jealous every time someone I knew was pregnant. Now when someone announces a pregnancy it's more like 'wow, I sure am glad that isn't me!!!' 

The decision to have another child is tricky, especially in a blended family, as you say, you are dealing with 1.5 instead of 2 kids in reality. Do you ever feel overwhelmed now? Would your son benefit from a full time sibling do you think?

I don't regret it one bit. It was tough to juggle everything in the early days but so worth it. I love that I got the chance to see the relationship between my two (bio) kids. They squabble but are fiercely protective of each other and are often found curled up together asleep in one or another bed. They're very much a unit. My SKs are close with my bios, it's just not the same bond....I think part of it is that they have less time together. 
 

2 sd's (18.5 & 11.5) had cut off contact with their dad about 1-2 yrs prior as a result of pas.

i had our 1st kid about 14 months into our marriage, fell pregnant with bubs 2 when my eldest was about to turn 8 months old. I was 29 when i married hubby and am about to turn 36 in a few months time

skids are not considered in any of my plans for having kids. 2 are adults, the youngest barely has a relationship with my husband and is about to turn 15 from memory. 

my husband says only 2 people are involved in plans to having kids, the future bio mum and bio dad- no one else. I miscarried earlier this year just when outbreaks of covid just started a few weeks prior. We may have another one after covid but financially hubby can have another because he earns alot of money.

OP,  you are way overthinking this. You've tied yourself in knots.

You want another child and have the financial means to do it  - have one.

I always wanted to have two children and this was part of my plan before I even met my husband. My SD did not factor into my decision very much at all. However we don't have 50/50 custody and I'm sure that would change things.

one of the biggest motivators for me wanting to have two children was the fact that I didn't want my daughter to be an only child. I know that technically she has a sister in SD but I know that due to the age gap, custody schedule and her general personality, I knew that they would never be close. All I could think about was holidays in the future wear SD goes to a big family event with everybody on bm's side of the family and her other half sister by BM, and my daughter never being invited. if something were to happen to me and my husband she would be all alone in the world. I wanted to have a second child for my daughter more than for myself.

When considering to have your own children, don't include the steps in the equation. You may have the perfect relationship with DH right now, but picture you two split, do you really want only 1 bio child? And do you want that child to only have 1 half sibling or more half siblings from either of you two? I think him having a full sibling is important. If you got pregnant right now, your son would be 3-4 yo, which is perfect timing to have a baby. He'll maybe in preK a couple days of the week and be able to "help" (put the pacifier in their mouth type thing).

I made a thread similar to this recently and got some good advice. My situation is slightly different since we'd be going from 3 to 4 kids (3 bios, 1 SK). IF our situation were different, I'd have another. To answer your Q about going from 1.5 to 2+, my bios are 16m apart and while it can be crazy at times, I really enjoy it. Of course are a little different when my SK is here, because like most stepfamilies, there's some adjusting the first day or so and there is an age gap, so interests aren't aligned. But my youngest is just now starting to try to crawl and interact more with my oldest bio, and so they are almost able to play together.

The logistics of 2-3 isn't really that much different, either. Especially close in age like mine-- I mean yes doubling up on carseats and all that, but we already had all of the baby stuff at the ready, and the house was still child proofed, and you're already accustomed to the sleep schedule of a young child, and your purse is always snack ready and all that---so going from SK to my first bio was WAY harder than bio1 to bio2.

No one can tell you whats best for your familiy situation except for you, but I know for me personally, stopping at 1 bio wouldn't have been what I wanted. 

What are your husband's reasons for or against? You say he's on the fence as well. Can you have a deep conversation about his desires or fears about another baby?

I can't get entirely on board with the advice not to consider SKs when deciding on adding another baby. SKs are a part of your husband's family, at least, and the decision to have more children is as much his as yours. (Using the generic "you" here, not singling out you OP.)

DH shouldn't argue that *SM* already has [SK + BK number of] kids, and bait-and-switch reneged promises to have more kids is a divorceable offense IMO. But... resources are finite, and family size requires compromise, especially in blended life.

SS wasn't part of the equation or decison-making when we decided to have two. If you want another, have another. It's important for a child to have a full sibling with whom they grow up and share a house 100% of the time.

I understand why some people say don't include the SKs in the equation, but you should include your steplife in the equation.

From the sounds of your steplife, things seem pretty solid. However, Disney parenting, alimony/CS for another decade, SKs who are mean/violent, an ex who causes ACTUAL trouble - all those things impact your marriage. Those things need to be considered when thinking about expanding your family. The last thing anyone should want to do is bring another being into a volatile situation that that being can't escape from. It's one thing to have kids when you're happily married and then your relationship deteriorates. It's a whole other bag of crap to purposefully bring another child into a bad relationship/marriage/situation just to fulfill selfish wants to procreate because the adult can leave while the kid deals with the fallout.

Again, it doesn't sound like this is your situation, so you can make decisions more from the heart than the brain. DH and I are actively trying for our first, and we had to delay due to financial issues. We're now playing the waiting game, but we've had conversations about how many kids we'll have. The reality is that I'll be lucky to have one, but if we get lucky enough to try for two, it'll be rapid-fire babies. Good news is that we're 4 years out from YSS graduating and BM being cut-off, so rapid-fire babies won't be so bad. A few years ago, though? Rapid-fire babies would have likely ended my marriage. We needed time, and I'm very glad we took it.

I applaud your use of logic.  Emotion is not an effective decision making tool.

Far too often the feels are primary decisioning tools in the blended family world while logic and intellect are a rarity.