To me this is the crux of his understanding but not understanding.
He has weaponized the kids, that makes any conversation based in "bad faith".
IMO, if relationships are down to transactional natures, then a way to possibly build them back up is to build small "goals" for the couple to rebuild it.
One thing is this. You should state, I expect to orgasm X times in a week, and I should 51% come first. This has to happen for a Y time frame, and failure to achieve this, allows an opening of the marriage to allow others in.
There is no couples privilege anymore as the contract was made null and void by us both agreeing neither of us were enough.
If you have a gifted child (I call all people with autism "gifted" as they have unique mental abilities to dive deep and hard into topics.
The actions and how the families harmony balance is "is critical".
If he bails on this, then as you stated birdnesting is it. By not accepting the counter offer which included monogamous style action, he is accepting this.
Put the hard red line up, get the exercise mojo going on, and realize he is 1/4 of a man trying to share a bed with a woman that is 3/4 to 1.0 full. He has to learn to up his game across the board or realize you deserve as many partners as you need to feel whole.
I too have a dysfunctional "marriage" and relationship. I also am aware of the areas where I fucked up and was wrong, though neither of us has cheated and had sex (intercourse) with another. That doesn't mean it is happy or whole, it means it is broken with the opportunity to fix it.
My issues stem from real trauma, and real "shit" done to me. My personal failures is not being a confident enough human being to get the therapy to fix me in my 20's versus starting it in my 40s. The thing is all by myself I did it, and have been working on it for 5 years. I work hard every day to deal with this and work on it.
Why? To try to fix things one more time and offer my partner who was damaged and has her own issues the same offer you are stating to your partner. It will start like this down the road.
I fixed X, Y, and Z with A,B, and C actions over the past 5-7 years. I am trying to do what you want the way you want and how you want, but my needs aren't being met. We need to work on it so you can either accept my needs and help fulfill them with me, open up the relationship so you can have your redlines/boundaries met and still maintain the relationship, or we need to break.
Every committed relationship from 6 months to decades will hit this wall. "Every one", the only time one won't is if you find your soul mate, and you work on all the communication and have perfect and open and honest conversations with full transparency "and" each partner always makes sure to take care of the others needs first and their seconds in equal balance.
It seems in your fight, he is on the defense always, and is not being assertive in his needs and using the kids as road blocks.
He is not being honest, he is not being communicative, he is not being clear, conscise or direct.
Without this there is zero chance he will be able to generate the desire within his masculinity or strength from it, to show you with fierce passion that he loves you and wants you to the level you did.
I found this out through the work I have "and" am doing. Since he is older, he should make not to harness his male energy and focus it without wasting it. I have found that doing so rolled back the clock for me and I am feeling more like a teenage boy then the middle age man.
Be well, and prepare plan C if neither of the above happen. Which is to liberate yourself as is his inaction is "action" also ... You have given what you can and at some times you have to call it.
As a reference, my relationship is like a 25 year old scotch right now, at some point one has to break the bottle open and drink it and move forward or on.