At the time of writing, Buster is EOL in four months, 30 June, 2024. Bullseye is EOL until 2026 and Bookworm's EOL is 2028. For this entry I chose to go with Bullseye because I wanted to install Foreman on my backup server, which does not support Bookworm yet.
Although this is specifically written for upgrade from Debian 10, Buster to Debian 11, Bullseye, this can be applied to upgrading any version of Debian.