Legally you are in such muddy waters here that my advice is somewhat different to that I'd give someone with parental rights.
Firstly you need good legal advice from someone who knows this kind of situation inside out. I imagine it's more common with same-sex couples so a more specialist forum. You need to pay for this, don't think a half hour from the solicitor on the high street is going to cut it. You need a full understanding of your legal position.
Second, your emotions are absolutely valid, of course they are, but that doesn't really matter in the scheme of things. Vent on here by all means but the way you feel doesn't affect the fact that this is your child's parent and you have to do what's best for your child. Sadly, this might mean giving your ex the chance to fuck things up. OTOH they may be over whatever it was that made them leave their child and ready to play a more full part in their life.
Note that doesn't mean I'd jump straight to 50/50. They are a stranger to your child, like anyone in that situation they need to start with small, supervised visits and build up.
If you have a baseline of where you want to end up: Ex, let's start small and hopefully build up to every other weekend in a year or so. Then you take the heat out of things while also laying down a line. The aim unfortunately has to be to keep the ex onside as I believe legally they hold all the cards. I'm so sorry, you're in a terrible position here.