The charms will most likely make it into the game whether we like it or not. Here is a suggestion which I think is a good middle ground and limit the power creep.
Im probably not the first to suggest it but I think this deserves more attention.
It would only require that sunder charms set the resistance to a fixed value of 95% resist and that cannot be altered. The 95% could be adjusted to 90% if required. This change, combined with reducing the max resist of a few monsters would make it available for players with less optimal gear, to farm areas with immunes. Infinity would still be a better solution and because it would allow to lower the ennemy resistance further. It would limit the power of cold mastery, no more power creep. Lower resist wands are also a thing that can be expanded upon and that needs to be considered.
I think the 1/5 penalty suggested would not be enough, would make infinity less appealing and would still increase the power creep too much.