Ubuntu, when used as intended, is secure.
There is no “less-secure” or “more-secure” flavor.
All flavors are equally secure.
As Quortal is distributed as an AppImage, updates (including security updates) are not automatic. That’s a choice made by the Quortal developers, not by Ubuntu.
It might be useful to tell us what sort of software you consider essential, and what you consider “unneeded”.
Depends upon your usage, and what you want to customize.
Generally all flavors of Ubuntu --with the possible exception of Ubuntu Core-- are about equally customizable.
Whether you find a particular desktop navigable is entirely a matter of your taste, which we don’t know.
Ubuntu and flavors vary mostly in out of the box experience; but the base system being identical of course means essentially they’re the same - they’re all Ubuntu.
Ubuntu flavors offer a unique way to experience Ubuntu, each with their own choice of default applications and settings.
The community flavor packages don’t get the same attention from the Ubuntu Security Team (as they don’t scan universe or community repository packages unless you add optional Ubuntu Pro); so if you’re after highest security possible, you might want to skip all flavors, but flavors tend to be lighter; so I’d not expect a clear winner that stands out (Every decisions tends to come with pros and alas cons).