A Fireface 800 has a lot of channels.
One ADAT optical connection allows sending of 8 channels at single speed (44.1/48),
4 channels at double speed (88.2/96 kHz) due to channel multiplexing.
If you want to use more than the 8 analog channels of your FF800, maybe also the Mic Inputs in the front and the phones output, then you might want to look for another recording interface. The UCX II has only 1x ADAT I/O.
Then you should definitively look at the UFX II which is equal in quality and features to the UFX III.
The only difference is that it does not support MADI (additional 64 channel via multimode fiber cable).
It is also more equal to your FF800 in terms of rack format.
Using the FF800 as preamp and AD/DA converter via ADAT is easy to achieve and you do not necessarily operate the FF800 in standalone more. I would keep it connected via Thunderbolt and Firewire adapter as confirmed by hselters, that it is still possible to connect it this way.
Then you can operate the FF800 still perfectly using driver settings and TotalMix FX.
In the DAW/application you would use the UFX II and access the channels of the FF800 (as preamp/ad/da converter).
When working with TotalMix FX you have two choices
a) to switch between the two recording interfaces in one TM FX Window
and to select the interface in the pulldown menue in the upper right
b) to open another instance of TM FX and to select UFX II in one and FF800 in the other windows.
I think option b) is the option which gives you best comfort and overview.
If you save the workspace then also the window sizes and positions of the two windows will be saved.
After rebooting and opening TM FX the two windows should be shown as before in the last session.
I think this would be a perfect setup for you if it also fits budget wise.
Should the FF800 fail, then you have automatically a higher port capacity and more options with the UFX II.
Features like: Autoset, DURec, RoomEQ and crossfeed are also available with the UFX II.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub14