What about distributing via Flatpak? (honest question, I'm curious to see if it actually help lowering the cost of distributing apps for Linux)
Regarding audio, if you decide to target several audio stack yes indeed. I hope this get eliminated by Pipewire really soon. It's already the case as a end user (Pipewire replaces Pulseaudio, Jack and the userland part of Alsa) but I don't know if all use cases are covered yet. If I'm not mistaken Pipewire started from the need to be able sandbox multimedia streams in Flatpak , so I guess Flatpak might be indirectly helping here :)
Regarding hardware it's the same problem for any operating system though, testing with all hardware can be tricky for a small company.