I don't know the full solution myself, I would need to dig into this code and potentially make some deeper changes. Probably we need to add a select and/or active texture component to datablocks in the depsgraph, similar to what exists for geometry. I might also do more work on the Cycles side, so we can detect when the effective node graph changes.

I suggest to commit this to 2.83. Don't revert it for 2.90, but leave the bug report open at normal priority and change the description to indicate cases where it's still doing too many updates (having both a Cycles render viewport and a viewport with workbench textures).