> I have been running Ubuntu on servers and desktops for around twenty years
If you have been running Ubuntu for that long, and don't have much experience with other distributions, the stable distribution you will probably feel the most comfortable with will be Debian. From an administration point of view, they are very similar; not only do both use dpkg and apt for package management, but also small things like details of the structure of the configuration in /etc are very similar. This is because Ubuntu started as, and still is, a derivative of the much older Debian distribution. The only thing you might miss would be Ubuntu-specific stuff like snaps; and you might also get a bit annoyed by Debian stable having older versions of packages than you might be used to (it's a slow-moving distribution, more similar to enterprise distributions like RHEL in that aspect).
If you have been running Ubuntu for that long, and don't have much experience with other distributions, the stable distribution you will probably feel the most comfortable with will be Debian. From an administration point of view, they are very similar; not only do both use dpkg and apt for package management, but also small things like details of the structure of the configuration in /etc are very similar. This is because Ubuntu started as, and still is, a derivative of the much older Debian distribution. The only thing you might miss would be Ubuntu-specific stuff like snaps; and you might also get a bit annoyed by Debian stable having older versions of packages than you might be used to (it's a slow-moving distribution, more similar to enterprise distributions like RHEL in that aspect).