Open Core is not an official Apple solution. We are not going to try to "fix" the driver to adjust to whatever it might be doing to the system. If the native FireWire ports appear to have issues in this setup, the easiest and likely cheapest solution would be to get a supported FW card, possibly even second-hand.
Please mind that you are using an audio interface that was developed and introduced 20 years ago on a computer that may be almost 15 years old. While both of these can still be used, and while you can create an environment where they will happily cooperate, maybe you should reconsider your expectation of being able to run the latest operating system here. And if you do not wish to do so, but instead insist on using unofficial software to make that work, at least be prepared to deal with certain side effects and necessary workarounds. A PCIe FireWire card would be a really simple solution. I don't even know if a Thunderbolt solution would be supported here..
An interface like the FF800 can only work as well as the system it's connected to. No driver can fix or compensate issues introduced by an unofficial way of squeezing a modern operating system onto what is essentially legacy hardware.
Also, I'm not even sure what the exact issue might be. Consider that it could also have to do with the old Mac simply not being fast enough to run Monterey smoothly. And audio performance apparently wasn't something the Open Core developers even considered.
Regards
Daniel Fuchs
RME