There's not really any benefit of running Manjaro over Arch, it will only introduce problems over time. If you want a "pre-configured" Arch with a nice installer, go for EndeavourOS, it's great!
You can but there isn't a lot of choice, Octopi is pretty much the only other pacman GUI besides Pamac that's sufficiently fleshed out. All the others are either just package searchers or CLI-only.
And Manjaro also has the Manjaro Settings Manager, which includes the kernel management module and the hardware drivers management module.