Edit: Solved for now, going to use Flatpak Steam and Flatpak Lutris to automatically use recent Mesa versions from freedesktop-sdk.
I recently switched from Arch to Debian Stable and so far I've been able to get updates for everything I actually need to keep modern from Flatpaks, Homebrew, personal backporting, compiling from source, etc. It's going well, and I think that stable base + a dozen cutting edge pieces of software is a better strategy for a modern desktop in general than something like Arch, where the entire system is bleeding edge.
The one thing I'm not sure about is getting a recent Mesa version - right now it doesn't matter much, but the Mesa that shipped with Bookworm was already half a year old on launch, so it might be 2.5 years old by the next release. If I end up wanting a newer Mesa version at some point before the next release, is there a recommended way to do it? I've read that backporting Mesa doesn't work very well, so it's probably not going to be a backport in the future.
I've found this script to build a local Mesa environment and allow arbitrary applications to use it instead of the system version, and it seemed to work well in a VM with some tests. You can specify which Mesa version to build, so I'm planning on building the latest stable Mesa branch if needed (e.g. not an x.x.0 release).
It seems like this is the way to go, but I'm wondering if anyone else has dealt with this problem before, and what the recommended path to solve it is if so. As before, I'm not sure I'll need a new Mesa in the next 2 years, but if I do I want to have a plan ready that doesn't push me to move to Sid or a new distro.