It took about 2 days to make sure that absolutely everything worked, including turning stereo into 5.1 + 2, and adding LFE to the adjoining room speakers so they didn't miss out on multichannel bass. Technically, I could map the extra channels from a 7.1 or 9.1 source into their nearest 5.1 neighbors, but I left that part disabled because I don't have any sources with that many channels.
The most tedious part of the setup was proving that each source type was supported, and all the speaker mappings were correct. Test files are extremely useful and there's no one source for all of them. I spent quite some time finding these files for myself.
For the pipewire / pulseaudio channel mapping I can go into the boring details if you want
Oh, and I plug a Logitech Bluetooth audio receiver into the line-in on the USB 7.1 soundcard, so I can pay music from a phone or my HiBy R2 DAP.
About the Pi: it sucked for a heap of reasons. I found myself fighting stupid battles over and over. Switching to an x86 made all of those problems disappear immediately. Stay the hell away from the Pi, is my advice.
I don't have a remote actually, I use the Kodi Android remote, MusicBee Android remote and kdeconnect (for everything else).
[PCs often have idiosyncratic default channel arrangements]