Wow, basically everything you wrote about Manjaro was wrong:
It doesn’t need constant maintenance, and it doesn’t break. The whole point of it is to be a stable variation of Arch.
It doesn’t have a highly irregular update schedule, it’s quite regular — every two weeks. There are also updates for outstanding security issues which can come faster (as needed). Occasionally very large updates can take longer in order to weed out all the issues, such as the recent example with Plasma 6 — those are announced in advance.
AUR doesn’t “expect” anything, it’s a dumping ground where anybody can put anything. I successfully run about 100 AUR packages on Manjaro without any issues, but nobody can guarantee anything when it comes to AUR. It’s officially unsupported on Arch and every Arch-based distro. If you want to call it dangerous that’s fine (if a bit hyperbolic) but don’t blame that on Manjaro, it just shows that you don’t understand how AUR works.