I think you can do this with a Maths Calculation Field if you add a "valid duration" field instead of the exact expiry and make the exact expiry date the calculation - i don't fully follow your field names but might something like this work?
Would need a load of fiddling with, and this is all untested and just made up, but i think that's how i would go about it.
It may not be any good for your timezone but we could have a go at it at the end of Roundtable Meeting 2022 06 if you're keen maybe?
I will try it out and see where I get to
Unfortunately I won't be able to make it to the roundtable meeting but thank you for the invite.