I'm in the market to SBR my PS90 and I see 3 methods to go about it:
1-Buy a new CMMG barrel
2-Buy a surplus P90 barrel
3-Cut the factory PS90 barrel

Option 3 would seem to be the cheapest and easiest.
My question is:

Is there a particular reason to cut the factory barrel and thread it with the left handed metric threads, then use an adapter to get to 1/2 x 28?
Why not just thread it 1/2 x 28 and be able to direct mount suppressors and normal AR muzzle devices?
Am I missing something here?

Thanks

Yes, you're missing something. What you see on your PS90 is the barrel shroud. It has to be removed to get to the actual barrel. Every SBR barrel adapter/flash hider I've seen is threaded with the OEM metric threads and you have to have that for the barrel to mount into the gun. The barrel is REALLY a true pencil barrel.

1) barrel is threaded into the barrel shroud which provided the tension to hold the barrel into the action.
2) either drill or dremel the roll pin out on the flash hider end of shroud and knock it out.
3) unscrew barrel from the chamber end from the shroud
4) remover shroud but NOT the barrel. Take a small dowel rod and push the barrel out so that you retain the barrel springs in place.
5) insert new SBR barrel pushing out the dowel rod.
6) screw barrel into your choice of flash hider/brake/threaded for 1/2x28 suppressor barrel retaining mount.

You're DONE!!

The post above explains it quite well. You can't really see it until you remove the barrel from the shroud but it is indeed a pencil barrel. Think of the barrel and the shroud or muzzle device like a bolt and a nut, since the barrel screws into the shroud/muzzle device. There is also some play in the barrel which is normal.

What I did for mine is took a portable band saw and cut the barrel flush right behind the pin, then removed shroud from the barrel. You will want to buy a cheap 15mm wrench and grind it down some to fit the flats on the chamber end. Send it off and have it cut to a length of 10.4" and threaded M12x1 LH. Buy your choice of muzzle device and reassemble.

I'd get the CMMG barrel, quite happy with mine and was an easy swap once my F1 was approved.  It wasn't terribly expensive all things considered.

Thanks for the info guys.
Got some paperwork time before I have to get started and pick a method.

Mark at tros cut two barrels for me

Second for TROS, they did a great job on my PS90 SBR barrel. No issues.

OP, I'm glad you asked this.  I just got a PS90 and I'm planning the same thing!!