On my Pixel RCS read receipts are optional. I imagine that’s the same for iPhones but I am not 100% sure.

I imagine on macOS it’s the same; Messages or nothing. No separate clients are involved.

Also they’d need 6 programs, the 5 they had before and now RCS.

iOS RCS works through the stock Messages app like iMessage does. It’s not a separate program.

I certainly do. iOS users insist on SMS and SMS is awful. I gently sway those I can influence to modern messaging but that is not everyone.

Read receipts have yet to work. I’m not sure if they’re off by default or it’s a bug in the RCS implementation.