You cannot add any more Sonos speakers to an Arc/Sub/Ones 5.1.2 home theater setup other than a second Sub. You could group a Play:5 to your current setup but it wouldn’t technically be part of your 5.1.2 system. It would just be an extra speaker in a separate zone. And in TV mode, the grouped Play:5 will have a slight audio delay.

Thanks, a buddy was going to sell me his Play 5 Gen 2 for $200 .  Maybe I’ll use it for the masters

$200 is a great deal for a used Play:5, and a master bedroom would be a great home for it.

Unrelated, but does the amp really make a difference in sound based on my setup? Just want to justify another $700+.   Seems like I’m just getting drawn to buying more Sonos LOL.

The Sonos Amp is not required for your setup. The Amp is needed if you ever want to add wired, passive third-party speakers to your Sonos ecosystem like outdoor speakers, bookshelf speakers, or in-ceiling speakers.