Having read all your posts, what I see here is two people who would be happier co-parenting from different households.
That is, you would be much happier. He'd be ok and live in his own filth but no-one would care, him included, so he'd be fine too. He'd probably be a bit whiney and very self-pitying but, as you'd be able to close the door on him, that would be ok too. You would have to be prepared for him to swooped upon by some self-sacrificing, rescuing angel, consumed with a desire to mother grown-ups, far sooner than you'd find someone else. If you were confident in your decision that you were happier without him, that would be ok.
He is plainly, as you say, an immature man-child but you, you're a bit of a martyr and marching determinedly down the path of adopting martyrdom as a full-on lifestyle. You also have a lot of internalised misogyny, which you are already passing on to your tiny children. Stop using the word 'nag' for one. It's a vicious word used only against women. Teach your DC about doing the right thing, making good choices and telling the truth, (taking responsibility for their actions, when older). All the beautifully neutral, positive, non-judgemental, child-appropriate terms that are widely used in childcare and school settings, children's books etc.
You portray yourself as thoughtful and accommodating, as quoted below. My question to you - and you to him - is what thought and accommodation has he put into surviving lockdown as a family? (Not 'with a family', 'as a family'). Where do you work together on this? Where do you and he compromise?:
I keep trying to tell myself to be less critical and less arsey
My poor children.... do I want to teach them resiliance and kindness to family members? Or do I want to teach them to be brave in the face of difficulty when it comes to putting yourself first and allowing yourself peace and happiness?
The passage below just makes me cringe. You're mistaking normal, genderless, pre-schooler behaviour, for some sort of adult flirtation and teaching that to your little boy. Yuck.
(And yes, people with similar attitudes, raise their little girls to be needy, manipulative princesses, by encouraging them to trade on their looks and cuteness).
And this is what my toddler DS is already doing to me... he's only little, but he already winds me right up then wins me over with a little twinkle in his eye!
I lectured 3yo DS the other day when he was deliberately making it difficult for me to put his shoes on him, "Stop being such a wee chav, because you'll end up marrying someone like me and they will find you really funny until they don't and then they will nag your head off for the rest of your adult life". DH overheard me and burst out laughing. Oh they both know what they are doing, the 3yo and the 35yo.
You (and your DH) have a choice about whether to raise the next generation of dependent, disrespectful, slobby man-children - or not. The power is in your hands.