control external monitor sound: if your monitor supports DCC, MonitorControl is FOSS and allows direct control. Also, see below.

capture sound: there's a flury of apps that allow you to do that, Rogue Amoeba has a couple that go from simple to very advanced audio routing. This audio routing also allows you to set the output volume (so, indirectly volume sent to a monitor) using the standard macOS controls.

> And yeah PulseAudio is just so much better than everything macOS can offer.

Someone has never dug into Audio Midi Setup, which can do a bunch of nice things already, including creating aggregated audio inputs and outputs.