@elena my initial thoughts when I here someone calling themselves a sysadmin is that they do system administration as their day job. One could of course be a sysadmin on one's spare time.

If one feels like they don't know enough (because they don't have sysadmin as their dayjob), one could maybe say "I'm a self hoster".

But a self-hoster or "hobby sysadmin" might know more about sysadmin stuff than one who works as a sysadmin.

Just rambling here. I think you could call yourself a sysadmin.