Upgrade Buster to Bookworm or to Bullseye?

So what now?

That depends on why you are running such an old distro in the first place.

For instance, do you have specific compatibility issues that you're trying to work around? Or is it just resistance to going through with the upgrade process?

If you're not constrained by some sort of compatibility thing, the obvious choice is to just do both upgrades now, so you've got that out of the way and don't have to do it again as soon.

(Plus, Bookworm is a better distro! Both because of two years of improvements and in terms of compatibility with software designed for modern systems.)