If I were in your situation I would contact MOTU support. I mention them because they've been around for a LONG time and were Mac-centric until recently. They also have excellent customer service.
I used to run an original 896 Firewire 400 interface on an old Mac laptop. It was the last version that could run either System 9 or 10, got left behind a LONG time ago. It is gone, I killed the drive and gave it away.
Chances are you could find an original model, or an 826? ( I think that's right?) for pretty cheap. MOTU may still have the drivers, deep in some dusty quadrant of a floppy disk somewhere.
I do have a couple of Firewire 400 cables still laying about it you need one. Pay the postage and you can have. PM me.
I hope and pray that I am done with the old world. Thunderbolt 2 is officially old but it rips along nicely and there are adapters for Thunderbolt 3/USB-C. Cheers, Kuru