I used Cache Cleaner from F-droid to clear all applications cache (both user and system apps). Since then my battery life is almost back to normal : I can use my phone without specific restrictions for 3 days straight.
It is still slightly less than what I used to enjoy (I consistently had 4+ days, not for every full charge but quite frequently still) but it is satisfying anyway.
Since the Murena mail/account problem is still not fixed, I still have the constant notification spam I mentionned earlier. I’m quite convinced that this is the culprit for the remaining battery life I’m missing and that it should not be a problem once this is fixed.