I can see you are becoming somewhat understandably defensive because it can feel like people are starting to pile on.
In practice, you are both being unreasonable. You have abdicated all responsibility for family finances and don't have a clue where the money is going and your husband is also spending above his means by the sound of it.
The reasonable thing to do is to sit down with him. He's asking a huge thing to sell a capital asset which may do you both no good at all. Before you both take such a big decision you need to go through your finances with a fine tooth comb - where is the money going today? Where are the opportunities to cut back?
In preparation you can take a look at your own spending which if you mostly use apple pay should be relatively easy to track if you can say that I took the £400 flat rental income and used it for nappies, wipes and days out. If your kids were in full time childcare there's a cost to that, it isn't "free" if you are doing it.
https://www.moneysavingexpert.com/banking/budget-planning/
Then I'd build a budget that looks at choices - if you go back to work full-time, what does that do for you both. I know that the answer is that it probably doesn't add very much if anything at all but show willing to consider it. It is a HUGE pressure to be the primary breadwinner.
Your husband may come to his own conclusions re the car. Selling the house that is loss making might be an answer especially if there is equity in it.
Shop around for a letting agent if you have a renewal on the flat coming up. It's a competitive market and people are selling flats handover fist right now. The simple act of checking your current notice period, serving it in principle and saying "I'd like a quote for next year" may drive a more competitive rate on the finders fee and the ongoing % charge.
Lastly, I'm not a property person but with three properties between you there may be an opportunity to remortgage in due course to reduce the monthly payments while the kids are little.
I do have one question - you are concerned about changing schools and wrap around care for your oldest child. Were you planning on only working 2 days a week until both your kids start secondary? If not, a move is coming at some point. If you are, then an open discussion on whether your husband is comfortable with that or whether he'd rather you work full time, the kids are in childcare with a split bill between you and he starts doing a lot more domestic labour including drop offs and pick ups. Be neutral, there are choices for you both here and you may find that the very discussion drives different behaviours.