If you need to spend some money for 8 additional analog port today, the question for me is also, what you might require in the future, whether it would perhaps make sense to spend a little money more for a better concept, that even has some room for more expansions and some more RME features. Some of the solution would even offer to record the same amount of channels with even higher sample rates (88.2/96 kHz).

Therefore a few thoughts, what might be all possible, finally you judge and decide.

- Digiface USB or RayDAT (PCIe) as a basement with 4 ADAT I/O, then connect devices in standalone mode via ADAT
- 802, UFX II, UFX+ with two ADAT I/O to add 1-2 AD/DA converter

Or a combination of a Digiface USB or RayDAT and 3rd party AD/DA converter like Ferrofish Pulse 16, which would support even 88.2/96.
Or a combination of 802, UFX II (with 2x ADAT I/O) and the Ferrofish Pulse 16).

A cleaner (ok, more expensive) approach offering more port capabilities even at higher sample rates would be a MADI or AVB based solution. MADI based I would think of the following devices, where it depends on what the base device / recording device with TotalMix FX shall bring on top (I think of FX, AUTOSET, DURec, Mic/Inst Inputs, ADAT, phones outputs, ...) and not to forget which interface technology to the computer ...

- USB2 based: MADiface USB/Pro
- USB2+USB3+Thunderbolt based:UFX+
- USB3 based: MADIface XT
- PCIe based: HDSPe MADI, HDSPe MADI FX (the only PCIe card with FX chip)

... combined with either RME AD/DA converter or 3rd party (i.e. Ferrofish).

IMHO the best, really best, solution giving you a lot of options more would be:
- new or used UFX+
- and using your UC in stand-alone mode (route analog I/O through ADAT)

Especially as the price of the UFX+ dropped significantly.
Then in the future sell UC and get a 3rd party MADI based AD/DA converter to stay "more in budget"
like the Ferrofish Pulse 16 M or of course take one of the RME products which support MADI.