RAM usage always grows to almost full. That’s how caching works. You should consider “buffer” and “cache” usage as “free”.
Could be. Even with a lot of RAM, a process that randomly dies is quite often because the OOM killer killed it.
This looks like a normal shutdown. There was a long thread about a problem that looked exactly like this one, which never came to a satisfactory conclusion about cause (but memory is my best guess). MariaDB error with mysql is not running on system
I probably wouldn’t try to get anything useful out of that conversation, though, as it was very chaotic and has a lot of confusing guesses, theories, and troubleshooting attempts. But, in the end the OP disabled some problematic WordPress plugins and the problem was resolved (which points to memory, indirectly).