Any Linux distro can have MySQL or Postgres installed in less than five minutes and works out of the box
Even a single core VPS can handle lots of queries per second (assuming the tables are indexed properly and the queries aren't trash)
There are mature open source backup solutions which don't require DB downtime (also available in most package managers)
It's trivial to tune a DB using .conf files (there are even scripts that autotune for you!!!)
Your VPS provider will allow you to configure encryption at rest, firewall rules, and whole disk snapshots as well
And neither MySQL or Postgres ever seem to go down, they're super reliable and stable
Plus you have very stable costs each month
I would have concerns around backups (ensuring that your backups are actually working, secure, and reliable seems like potentially time intensive ongoing work).
I also don't think I fully understand what is required in terms of security. Do I now have to keep track of CVEs, and work out what actions I need to in response to each one? You talk about firewall rules. I don't know what is required here either.
I'm sure it's not too hard to hire someone who does know how to do these things, but probably not for anything close to the $50/month or whatever it costs to run a hosted database.