There is a lot to thinkabout, and only you can essentially gamble with the chances.
I would say move forward with the embryo implant. There is very chance it might work out, and that is what you had planned and wanted and hoped for. I would feel like I deserved to let myself do that if I had planned that opportunity for myself, and I know that were I to interrupt that to do something with potentially less successful outcomes like a freeze all, and it wasnt successful, I would find it hard to not be incredibly angry and resentful were things not to work out with the sibling placement.
I would shift my thinking to believe that it is the social workers who make the decision about whether your family is right or not. There are many worse things for an adopted child than having a sibling who is 9 months younger than you, and although in adoption terms it is not really recommended, sometimes these things can work out if the effort is put in to how sustainable it can be. However, I would be of the opinion that it is not really your job to decide if the gold is placed with you if your heart is open to it, rather that it is the social workers choice as to whether they want it. You can know you offered a placement and were committed to making it work.
I say this in the nicest way, but society is set up for a family of four, and to move to a family of five (I'm the eldest of 3) meant things like we had to get extra hotel rooms, were always crowded in the middle seats of the cars, had to buy two packets of 4 bread rolls, always be outnumbered etc. I felt some resentment because I was frustrated that we couldn't just be 'normal'. Finding babysitters for the three of us was hard, holidays are set up for 4 people sharing, all that stuff was difficult for my parents, and because my two you gest siblings were close in age, my mum's maternity leave ran quite close together so her career didn't really survive that and having two young children as the childcare costs for both to be cared for were extortionate, and by the time it was financially viable to go back to work, she had been out of the market for several years.
I mention this not because I think it is the wrong thing to do, but because in many ways, if you do turn out to have a family of 5, your narrative can very much focus on how lucky you were, how many stars must have aligned to make it happen. What a happy set of chances it will be if your family is created because the BM pregnancy goes well AND their social worker places the child with you AND your pregnancy goes well.
I might be worried that if AC was placed and then your own pregnancy didn't work out that the grief might become entangled with the early childhood of your AC. You couldn't know that in advance, and even were that to be the case, it isn't the end of the world but in those early days the bond is so fragile that I would worry about it being a trigger for post adoption depression, and in some ways thinking that the stress of the adoption process contributed.
There's loads to consider if you do have them, and your mind is possibly spinning, but should it go ahead, the below is what I would be considering:
Having two children very close in age is both a blessing and a pain in the arse. You need to think about loads of stuff, not just when they are babies but further on in life too. For example, as long as BM doesn't go over term, that child will be summer born (very summer born), so they won't be in the same.school year, but unless you want them to be in the same class (which I wouldn't recommend), you won't have the option for deferral, even if your AC has any delays.
Financially you are also on the hook for lots of things close together - university fees, school trips, but you can also do plenty of hand me downs, although if AC is in any way delayed that might be trickier. Also, my eldest hates giving stuff to his sibling, and the rate they grow I can't even hide it until he forgets!
There is also the gender thing, and I say this as someone who didn't give a hoot about gender before I adopted. We have two boys close in age and it's actually more unusual than I thought and draws a lot more comment. I may be biased, but they seem to spend half their lives wrestling, jumping climbing and shoving, winding each other up and being generally socially unacceptable, and whilst in some ways I can just roll my eyes and brush it off to onlookers as #boymom! In other ways I think it would be different if we had one of each as their interests and the reactions of others might be different. Both of my boys are very different kids, and we do encourage them to be, but people do bracket them together. This has done wonders for their sibling relationship, but equally means that they expect the boys to react the same and they really don't, which can make it tricky to get the right support in place for them or avoid comparisons between them, which may be tricky for an AC and BC in the long run. (And before anybody comes for me about boys and stereotypes and gender norms and it not mattering, before I had my two terrors I would have said the same thing. Turns out life's favourite thing to do is give two parents who were dedicated to breaking gender norms two boys who love dinosaurs, mud, guns and swords so much that they can name different 16th century cannons at 4 years old and insist I read every pissing noticeboard at the transport museum to them). However much I hate to say it, other people's comments will be amplified and more prominent if you have a less usual family make up, and I can imagine that having 2 children so close in age would be a magnet for comments and questions, probably just as you are trying to get them all in the car at the supermarket and they are all kicking off.
Anyway. I would query whether the gender make up change anything, or the dynamics when combined with age? I would be aware that your current AD would feel like the outsider from "the babies", and each combination may mean parenting those combinations differently. If the two adopted children were girls, would you expect them to bond differently? How might that leave your son feeling? If the two youngest were both boys would that leave you AD feeling left out? Would 3 girls have a lot of competition or even possibly make them feel that they had to be "the sporty one" or whatever to stand out? Etc etc
All things considered, and knowing nothing about your situation beyond what you have told us, I would be inclined to go for the "shoot 'em all let God decide" approach. There are too many variables in both options for either to be considered a sure enough bet to exclude the other, and as long as you are genuinely prepared to be a larger family should all the moving parts come together, a very interesting story to tell in the future.