I'm gonna be 100% here, .45acp is a highly unusual cartridge for ARs. If you want a quiet pdw you'd wanna look into 300blk, if you wanna make it bufferless you're going to be hunting into proprietary territory. BCA has a bunch of bolt-action 300blk uppers and that would be a good place to start. CMMG has some for 9x19 and also a great place to start.

Don't dismiss the buffer though, having a nice long stroke is super smooth and a collapsible stock on a carbine buffer tube with ~8" barrel is surprisingly short. Even a fixed stock with a shorty barrel is pretty compact.

Same upper with a collapsible lower

I don't think that actually exists yet. You could get a bufferless lower but then you would need to buy/design a magazine adapter. Alternatively you could always use this and then just screw in a cap instead of a buffer tube. https://guncadindex.com/detail/HT-D3D-AR45-10-V1:3