One of my previous cars had a constant fault like this that my mechanic couldn’t find the cause of, even after a new battery was fitted.
Anecdotally, I’ve had one of those. Half the time it’d start perfectly, otherwise you’d get “click” and it was dead as a doornail.
Long story short, weeks of going through several mechanics and auto-electricians failed to diagnose a faulty rectifier on the alternator. Ended up taking it direct to Lucas who found and fixed it in under five minutes.
Dunno as that helps the OP though; sounds like a fault on one of those subsystems mentioned is draining the battery.