For some time I've been a satisfied user of Rogue Amoeba's SoundSource as my playthrough app, but in their latest version (4) they've removed the playthrough feature. I had a quick chat with them and they see it as a niche thing that isn't core to the app, which is fair enough.
It's an important feature for me though, and one I don't want to lose. I've been playing around with CAPlayThrough and built most of an app around it, but I have some issues with the CoreAudio parts (mainly it drifts over time) that I'm not currently qualified to solve.
I'd be thrilled to collaborate on building a simple app for doing playthrough on macOS - would that be of interest to you?