I am not an expert on linux or surround but I have gotten 2 setups going recently with linux and surround sound.
I do not have a Pi with me now, but going from memory when I set up the Pi running raspberry pi os with MC I did the following:
In MC/Options/Audio/Audio Device choose the correct device for your dac, there will be many options with "motu" in the name. The one that works for me is “Plughw:....[ALSA]... Hardware device with all software conversions” Where I have dots you should have the DAC name. Make sure it starts with Plughw, and ends with All Software Conversions.
In MC/Options/Audio/DSP & output format set your channels to 5.1. Be careful, sometimes this reverts back to stereo if you play something to that zone with the dac disconnected.
In raspberry pi os right click the volume thing by the clock in the taskbar and choose Device Profiles. For your motu dac change it from stereo to Pro Audio (I believe it Pro Audio, it obviously shouldn't be stereo though). Also make sure the motu is selected as default device there. You could turn off the other devices.
When I first setup the Pi I had no sound. Switching from Pro Audio to Stereo, then back to Pro Audio fixed it.
That's it.
If you still have issues you could use VLC to troubleshoot. This could let you know if it is an MC setting issue, or a linux/dac issue. I don't use VLC much but, but from memory, adding the dac it is essentially the same as MC. In VLC check the box to show advanced settings, then choose the sound device and set to 5.1.
I have never messed with pipewire, pulse audio, or ALSA. I just use whatever is included in the OS and things work. I have used Kubuntu, Raspberry Pi OS, and Fedora KDE for surround, with miniDSP Flex HTx, HiFiBerry DAC8x, Behringer U-Phoria UMC1820 USB Audio Interface (UMC1820d1), and ASUS Xonar U7 Sound Card.