I just built my first 9mm AR last year, got to admit they're pretty fun. Before ammo decided to pregame for the coming election it was super cheap to play with which is nice. Personally I'm not a big fan of those magwell adapters, I like the dedicated lowers. I also know my luck, somehow I'll go to strip a mag and the whole F*ckin thing will come out with the mag somehow.
I went with a KE Arms dedicated Glock lower, never used their stuff before this but its pretty good. Ejecter came already set in the lower so I didn't have to mess with that, plus feels good and sturdy. Not sure what your options are in Cali for lowers these days. Upper is a complete off the shelf Foxtrot Mike I got from Primary Arms when they were blowing them out last year. Nothing but good things to say across the board about it so far, it survived getting beaten and thrown around the fall shoot last year with no issues.