It upsets me hugely that he is actually trying to lessen contact with them, and his current mental state makes me think that perhaps this is not such a bad thing and that I should be asking for a psych report and considering possible full custody. I don't think this is ideal for the children, but like you realise that my ex's behaviour is abusive.
Frankly, you can't force someone to be a more involved or better parent. You can have that door open and not block it but in the final analysis it's his choice. And yes you're likely to get unfairly blamed for his failure to parent.
Probably you will just have to find a reasonable balance and try to limit the amount of erratic inconsistent behaviors, blaming and blame-shifting that he may do. Deal with what IS, not how you wish it to be.
Going forward, do what's best for yourself and the children. If he can be a positive and significant part of their lives, fine, if not, well that's the way it will have to be.
OFW does get positive marks, but as Nope wrote, "What OFW seems to do very well is
document." So much of what happens in post-separation life is ignored in court as "he-said, she-said". OFW enables the court and other professionals to view what is actually happening or not happening. And while it doesn't enable a parent to falsely claim "I'm not being informed" it does prove whether it's true.