I did some searching and it looks like multi channel FireWire audio can work well on the M1 Mac running Big Sur.
RME is offering macOS 11 FireWire drivers for some of their legacy interfaces.
That said, it might be an expensive process to rewrite the Tascam drivers.
(Or maybe easy for a dedicated 12 year old.

)
I'm wondering how many DM mixers with the firewire card are out there and then, how many of those are folks are running MacOS.
Maybe take up a collection and hire a programmer? (sorry, just a dream)
Or talk to Apple and ask for advice.
They are sometimes sympathetic to solving problems like this.
My take away so far is that it is very possible to accomplish the task.
It would be great to continue the legacy of the DM mixers with Apple's latest machines.
Fingers crossed.