I a non-expert may add his two cents:
> what are the critical Moodle folders and files I should save everyday in case of a calamity like hardware failure
N.B. 1. That result has any value only if you've done the reverse: https://docs.moodle.org/31/en/Site_restore.
N.B. 2. You can't be doing that manually every day. For that you write a script, shell script for example, which is kicked daily by a timer like the https://docs.moodle.org/en/Cron.
> not the Moodle in-house backup feature (too buggy and unreliable I think)
Would you reveal what in-house backup feature you mean and why do you think it is too buggy and unreliable?