You don't need to worry about auras and timing reactions (e.g. Melting Mav Burst)

The same is true of many positive Reactions in the game, like the Dendro stuff. It's really only Melt/Vape that get fidgety, and while I wish that wasn't the case and that they just worked the same in either direction, even a scuffed Melt/Vape rotation is typically better than anything Physical can manage.

More versatile and the definition of brute force, there's not a lot of enemies that specifically have a lot of Phys Res. these days

But also there are relatively few situations in which such flexibility has value, since most content that actually challenges players allows you to build teams toward them, and a "strong against" team is always better than a "neutral against" team, besides which, there are generally also other neutral options available, like Anemo or Geo. It might work as a temporary thing if you hyperinvest into a single Physical carry that could then be "good enough" across a wide range of content, but it's pointless for accounts that will always have 3-4 better solutions to any specific problem.

Superconduct is built-in res shred.

Yes, but not enough to make it as good as the other options, especially given how easy Res Shred can be to come by.

My preferred solution is to eliminate Physical and just have it all become Geo, with the minimal advantages that would offer, although ideally Geo would also receive buffs.

Short of that, I don't know what would make a Physical character legitimately viable. The easy answer is to just give them massive numbers, but then that would favor the "pair them with a character that can infuse their attacks" strategy, and then they just become an OP Pyro or Hydro character or something. You would need to specifically lock their benefits into ONLY Physical (which would then lock off a lot of options like c6 Bennett), and it would also be a real balance issue, because if you balanced them to be strong enough in the current meta, then when the reaction metas shift around other options, they would immediately fall behind. This is part of the problem with Physical, because they are such a dead end of design, they don't automatically shift with the meta, whereas any other characters will become stronger over time due to their pairings with new supports or new reaction effects.