I think your plan as written is (nearly) infeasible. I doubt that you will find such an accommodation. And you won't find a suitable salary even if someone wanted to take you on. The market is insane at the moment.
Instead, I would try to seek a position, with a short tenure clock (2 years, say). I'd suggest that you "offer" to drop in rank to Associate, but no further, and only if necessary.
I would, instead of applying through the normal system initially, write directly to department heads, explaining the fact that your current place is likely to fail. Write as a colleague, one professor to another. Ask what might be possible. They will have to make some exceptions to hire you in any case.
But to make even this possible, you would need to look fairly broadly and be willing and able to move. Don't limit it to just liberal arts colleges, though research universities might be out of reach unless you have an exceptional research record.
If you have kept up your connections through, say, conferences, collaborations, and such, try to exploit those connections to find some allies who can work on your behalf. If your own department head is an ally, ask them for help in introducing you to other heads.
Don't sell yourself short. Sell yourself as a potentially valuable member of the faculty.
If your current institution is primarily a teaching college, note that some very top institution (Duke, Stanford, ...) have a special faculty category, sometimes called Professor of the Practice" that focuses on undergraduate teaching. They do a lot of that and want to free up the research faculty. The positions I know of aren't tenured but, rather, long term contracts that can be very stable. They expect a very high standard as you would expect.
Edited to add.
The main problem I see is that a regular application for assistant professor is so "odd" or "off the wall" that nothing you could say in the cover letter is likely to overcome the strangeness of it. Offers for junior faculty are usually governed by rules that would make this idea preposterous. They are looking for "fresh" faces just out of grad school, not experienced people - either teachers or scholars. A few adverts are more open that the rank is negotiable, but if it specifies Assistant Professor level, they almost certainly want someone at the beginning of their career.
You on the other hand, can fill in a different sort of need for some as a mid-career experienced person, likely focused more on teaching than research.
In any case, I hope you find success, however you arrange it.
Thinking again, overnight:
You may have considered this also, of course, but you are at the perfect time in your career to move in to administration at some level. Many large universities, even (or especially) R1 universities might have a position like "Associate Chair for Undergraduate Instruction". Math and CS departments tend to be rather large and have lots of courses taught by TAs, for example. Someone with 20 years experience at a Liberal Arts college knows a lot about the curriculum requirements and, I suspect, quality teaching. There might even be some opportunities at the level of Assistant Dean for an experienced academic. A couple of the "Professor of Practice" people that I know have actually wound up with oversight of the undergraduate program.
Some people at the "chair" level even get to (or are required to) teach a course or two, if that is especially appealing to you.
Furthermore, accepting such a position would probably leave your academic rank unchanged, though tenure rules for administrators might be different. Often, though, administrators retain tenure "as faculty", even if not "as administrators".