Honestly, I think you're underestimating the reality of the decision.
I left my exH aged 30 because I was bloody miserable and he was EA (it was definitely toxic and brought out the worst in me too). DD was aged 3. I had no intention of entering another serious relationship whilst she was young or of having another DC.
However I met my new DP quite quickly after separating and fell completely in love. DP is fantastic with DD and doesn't have children of his own but it's still been tough at times for all of us making these transitions. I think had DP have had his own DC to blend too it would have been even harder.
My DD spends 2.5 days a week with her DF and I miss her terribly each week, even 3 years on. She was far more disrupted by the split then I anticipated (arguably due in part to her DF dealings with it) and is now much more adjusted but that took a lot of time and some sessions with a child psychologist. I spent the first year or two feeling dreadfully worried for her mental health and a lot of associated guilt.
Anyway, DP and I have decided more recently to ttc, but it appears we are unable. Partly this is very sad and upsetting, as we would both love to have a DC together. However in complete honesty there's still a part of me which worries about how the reality of this would effect DD had it happened.
What I am trying to say, is that I don't for one moment regret leaving EXH and am very happy with DP, a love I never expected to experience. Everything in my situation is as 'perfect' on paper as it could be really, but it's still been bloody tough and difficult at times. In my example there is also no guarantee of another DC, as for us it turns out that's just not happening.
I would say to anyone who is miserable in their relationship to leave, because life is short and you deserve to be happy. Adjustments are made by everyone and you muddle through, but don't underestimate how hard the reality of it is.