Loathe as I am to put details of my life on Mumsnet as a noisy minority seem to love nothing better than to judge or be rude to other women in a way they would never be face-to-face, I am older than you are OP and pregnant ☺️, solo mum by choice, 2nd attempt at IVF, honestly did not think it would work and was just doing it so I knew I had no regrets, thinking 'what if'. Time will tell if my child grows into a perfectly adjusted-faultless child who at no point resents me for choices I have made, unlike Mums who have kids with male partners.....
You're not going to get a balanced view from Mumsnet. All kids will end up with some beef against their parents, but the degree will vary. I knew years ago that I would rather have a child solo than with the wrong man - I have seen the damaging impact on children of uninterested, lazy fathers time and time again, and know I can do better alone. Getting back with your ex in the hope he's interested in some step-father situation when he may not have indicated this is very risky - you say you don't want to single-parent, you want a family - well your ex has told you he doesn't, not one with kids anyway. Risky. Only you can make a call on your current partner, loads of women go ahead and have kids with men who end up being not the best Dads and if some of those women hadn't gone into the situation with blinkers on, they would have been able to anticipate their partners would be rubbish dads-who knows whether your partner would be a worse or better Dad than all those under-par Dads....
Maybe take a look at forums where Mums have chosen to solo parent - often they say it's the best thing they have done, and many are pleased they chose to go it alone - though those that post are a selecting group ofc. I am not sure you are in the headspace to go this alone, from what you have said tho?
Either way, the one thing I would say, and that you well know of course, is to think quick if you can. Good luck!