If your decision is Firewire (which is not well supported anymore under Windows) then Chapter 10 of the FF800 manual gives you valuable information about how to operate up to three Fireface 800:
https://www.rme-audio.de/download/fface800_d.pdf
https://www.rme-audio.de/download/fface800_e.pdf
As of now there is at least a chance to get it working by installing the legacy Firewire driver under Win10.
From cost perspective and the operation surely a benefit.
Then you can operate all devices per TM FX and the driver settings dialog.
Using a Digiface USB gives you some limitations, because you can only route 8ch in singlespeed over ADAT. Its IMHO a "waste" of ports of all these FF800's (assumed you do not want to use analog ports (no Mic/Instr ports).
At the end you would need for such a Digiface USB based version most likely Firewire anyway, to be able to change gain settings etc.
So it boils down to the point, that you can't get rid of Firewire if you want to work with firewire devices even when deciding to use standalone operation because I assume you also want to use Mic Pres and then you need to control the gain. But even with line level ports you might want to adjust the reflevel per port ...
The only way out here is to use newer components and then you need to think about whether its worth for you to invest into MADI to get all the useful features like e.g.: Auxdevice Support through either MIDI or MIDI over MADI, Delay Compensation, Autoset maybe even DURec.
Best features you will have with a combination of UFX+ and XTC, there you can use even Delay Compensation and Auxdevice support immediately and have a variety of options to use the preamps.
More up to date converters and higher port densities with UFX+ and 12Mic. But Auxdevice is announced, not yet there, Delay compensation is not possible because 12Mic doesn't follow an 8-port schema. On the other hand Delay compensation is not too critical, as converter became faster, if you have stereo mics, simply ensure to connect them to Mic inputs of the same preamp.
What kind of ports do you need anyway .. before makeing more proposals ?
How many Mic, Instr, analog / digital ports ?
Do you want to be able to get special RME features (Auxdevice, Autoset, Delay Compensation, DURec, ...) ?
What is your maximum budget for that if you would e.g. sell the old stuff ?
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub14