I got my 2019 iMac to talk to my Firewire 400 Nikon Coolscan 9000 by:
Thunderbolt 3 (USB-C) to Thunderbolt 2 adapter
Thunderbolt 2 to Firewire 800 adapter
Firewire 800 at one end, 400 at the other cable
And it works! Not sure if your tethering software would work with such a chain of cables (and you probably wouldn't want to drag all that outside), but VueScan finds the scanner!
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