It looks like you've settled on a Glock pattern lower. I chose to go the way of a Colt pattern. I had no reason to go with Glock mags and I think the Colt style mags look cooler.
Just some food for thought. Some bolts don't play well with milspec hammers and require a 9mm specific hammer. Whatever that means. It's just an issue I ran across on the internet. I never experienced it. When I decided to dip my toe into the PCC world, I went the cheap route.
I have uppers laying all over the place, so I went with a mil spec upper and standard charging handle. I got a Faxon bolt. I went with this bolt because it is compatible with both Glock and Colt pattern lowers and works with a standard milspec fire control group. I also got a Ballistic advantage 5.5" barrel. I got a cheap free float handguard from AR15handguards and put it all together.
Since I was just dipping my toe, I got a few Endomag inserts that slide into PMag bodies. With the parts I've listed, the inserts worked 99%. They didn't like hornady hollow points. When I decided that I liked it, I got one of those inserts that slides in from the bottom for around $50 and just dedicated one of my lowers to 9mm duty.
It runs like a clock.
Oh, and don't tear your hair out about which buffer you should go with. Just go with a Vltor stainless A5 buffer. They are the correct length and less expensive than the insane prices some charge for a good 9mm buffer. The Vltor part numbers start with A5SH and come in different weights. A total of about 22 oz. for bolt and buffer is where you want to be. As for springs, I use a Sprinco blue although I know a few that opted for the red.
Honestly, I built it because 5.56/.223 got so expensive and 9mm was still kinda cheap when COVID hit. Now, 9mm is pretty expensive. I just ordered 1000 rds of 9mm and it was almost as expensive as 1000 rds of steel case .223. I don't think I'm gonna be shooting to much 9mm out of my AR in the foreseeable future.
I also saw a comment above that an AR9 has no recoil. I don't agree. An AR9 has more recoil than an AR15 in 5.56. Blowback vs gas operated.