I have been thinking about self-hosting my personal photos on my linux server. After the recent backdoor was detected I'm more hesitant to do so especially because i'm no security expert and don't have the time and knowledge to audit my server. All I've done so far is disabling password logins and changing the ssh port. I'm wondering if there are more backdoors and if new ones are made I can't respond in time. Appreciate your thoughts on this for an ordinary user.
You can't trust any of it to be totally secure, it's effectively impossible. But, this is true of all software, at least open source is being audited and scrutinised all the time (as demonstrated).
All you can do is follow best practices.