It has been reported before that a user can easily be overtaken by this warning. I think this is the problem to solve.
Loosely speaking, the phone requires some “headroom” to restart. So if that required headroom exceeds the free space then system will fail to start and Recovery will start instead.
(One reason the the phone might suddenly overfill could be that some automatic backup occurred.)
I think that trying to understand as much as you can of the “encrypted /data partition” thread which you linked above is the way to go.
Good luck.
- This post includes some more links. Samsung S9+ crash full device memory restart - #3 by aibd