You want to expand the inputs and outputs of the Mac computer, thats no problem with aggregate device when each interface works correctly alone with their drivers.
At 192 kHz the restriction is to the Adat connection only.
Instead of 8 channels at up to 48kHz, it provides only 2 channels at 192kHz, inputs and outputs. For adding an Adat device to the UCX ll this would be a restriction .
In your case when all units are connected to the computer with USB, all analog inputs and outputs are available in the DAW.

waedi I think I follow. I will try an aggregate device, hoping it does not affect performance.

Besides aggregating, unfortunately neither of my other devices has ADAT out.

For my own understanding, not my actual use... 192khz is a must for me:
Is RCA out > S/PDIF In a thing at 44.1/48khz? It seems the fact that RCA is unbalanced is an issue unless converted somehow?

And when does sample rate discrepancy create a clock sync issue? I thought I read this, but is that simply why S/PDIF and AES are automatically disabled?

Any device suggestions is good, too, (preferably less than $400).
I was looking at the Behringer ADA8000 but I only really need 2 inputs, and MUCH preferably Hi-Z, but I can deal with Line In.

Twin Duo:
Scarlett 2i4:

Thank you for the help

I will try aggregating tomorrow.

That Swissonic…
can I use that with the UAD Twin? convert TRS to RCA? Then connect it to the UCXII optical in?

Thank you again!

Direct monitoring in the Totalmix is no problem. Just do the routing.
Yes clock-mode external SPDIF is correct when connected with the RCA
If you use the optical connection then clock-mode external optical is to use.
Edit : None of your interfaces has an RCA digital input so you have to use an optical toslink cable and set clock-mode to external optical adat.

I’d suggest it needs to be standalone and you need to set it so that it mirrors its inputs to outputs. Have you read the Apollo  manual to see if this is possible?