I would try and not seem too confrontational about this so as not to alert him. Just say you understand but you don't want to do this with him and need to disentangle your finances first then if he wants to buy it he can.
But when I read about him I see arrogance. Breathtaking arrogance of the type that men who think they have it all display. He feels like he's on top of the world - you will of course go running back. Change the locks ('lose' your keys) and get a formal separation in place.
Be prepared for him to get VERY nasty. He will stop paying the bills so you need to get the house on the market straight away. With a bit of luck that will at least keep him paying the mortgage. Because you can see a solicitor but if he pays all the bills currently and decides to stop, telling the world that you kicked him out and that's why he's not paying a solicitor can't force him to.
So that needs to not matter to you. You need to be financially independent. Half of any money in joint accounts is yours and I suggest you try to get the logistics of the split pre divorce organised with him first.
Factually explain it's over. You want to be fair, the kids will come around (you actually need them to as he will go bananas otherwise and stop paying) and you will sell the house. So that means no debt in the meantime and he needs to support his children. Keep all emotion out of it.
Meanwhile a fucking hot solicitor will explain that you are entitled to at least half - and you need to get all his documents - pensions the lot. And put half of the joint account money into your account - tell him you are doing it (after you have) so he can't use it against you.
But I think this is going to be a bumpy ride. And he needs to feel like any move he makes is public. Not looking like a cunt to others is probably the biggest thing stopping him from being a full on cunt.
It's going to be horrible. One minute you will feel all girl power the next minute like a 5 year old girl yourself. Gather your friends around and fuck the Covid rules about having friends over (or get great outdoor heating) and stock up on the vodka (highly recommend both Aldi and Lidl for cheap yet exceptional booze)
Start thinking about work and how you can step that up. Take up exercise. Start putting yourself first.
And encourage the kids to see him (not in your home!) otherwise he will try that spare room shit again. 