You expand via ADAT1 or ADAT2 out of your UFX II
ADAT HW Outputs in TM FX have a fader .. so yes you can of course control the volume.
And even more, you can create a fader group to control volume for several outputs at once.

You could consider to take a RME UCX, as this is the cheapest recording interface which has 6 programmable standalone profiles, where you can hardcode the routing very easy for standalone operation, which is tad a bit less expensive.

But tbh I would use the ADI-8 DS its the proper high quality "tool" / device  for this task:
https://www.thomann.de/de/rme_adi8_ds.htm

Which one of the two now ? The converter or the recording interface ?

Look into the handbook, usually you can use up to 2 or 3 interfaces connected to USB in parallel.

As you use then the same ASIO driver, the DAW can access all three at once.
There are only 2 requirements
- clock synched (you could either "waste" ADAT) or use WC
- both interfaces need to use the same ASIO buffersize

The drawback is, that you need to open / work with 2 different TM FX instances and you can't have routing across TM instances.

Therefore I would regard it as better to expand by using the ADAT channels which are there for such purposes and invest into a real ADDA converter that directly connects to your one UFX II.

In terms of handling its easier to work with one TM FX instance and
in terms of functionality its better to have any to any routing capabilities in the one TM FX instance.

I would recommend you for those reasons to stay with 1 UFX II and the additional converter.
Its not so much difference in price but better functionality / easier and more straightforward to use.