DO NOT migrate / upgrade anything to the snap package that package is from Canonical and it’s after the Incus fork, this means if you do for it you may never be able to then migrate to Incus and/or you’ll become hostage of Canonical.
About the rest, if you don’t want to add repositories you should migrate into LXD LTS from Debian 12 repositories. That version is and will be compatible with Incus and both the Incus and Debian teams have said that multiple times and are working on a migration path. For instance the LXD from Debian will still be able to access the Incus image server while the Canonical one won’t.