1.- Can I upgrade the server, without damaging anything, and/or can I install composer?
2.- You can also update the operating system on the server, without problems with Hestia, on Linode, which is the platform I use. I have Debian 10 and I would like to update it to Debian 12.
Be sure to upgrade to Debian 11 first. As long as you know how to upgrade Debian, you should be fine. I have a Debian 12 HestiaCP instance that began life as Debian 10.
Bonus Tip: when asked if you want to keep or replace any configuration files, be sure to keep them.