Interesting. Would such a person be able to change the rules so presidential pardons entirely void whatever they're pardoning?

I don’t think he needs to.

they will appeal based on the insanely broad immunity ruling and get the conviction overturned on procedural loophole created by the ruling - no evidence from his presidency even for unofficial acts and justice department will not retry a sitting president.

He is likely to get the convictions even in the caroll defamation case for the same reason.

Only the fraudulent valuation case is likely survive this Supreme Court ruling.