"Is there a beta driver working for OS Big Sur yet..."—no. Although I think the old kext driver still works under Big Sur, as it does under Catalina (this requires somewhat tricky enabling of the deprecated support of kernel drivers).
"if not yet any ideal when we can expect one ?"—never.
The products have been discontinued for years, and are no longer supported. It doesn't mean there is no way to interface the mixers, but it's definitely the end of the road for the firewire interface. You could, for instance, get a class-compliant USB to ADAT optical interface bridge, or a Thunderbolt or USB audio interface that has ADAT optical i/o.