to 3)

> [ UCX and UCX II  ]
> My understanding is that since they are of the same product family, TotalMix will allow me to use both units in Windows.

It's not something like "product family" which is the relevant part here.
The point is that an application for audio (DAW or whatever) can only load one sound driver at a time.
So, to be able to use two devices, they need to use the same ASIO driver under Windows.
This is the case with the UCX and UCX II as they are using the same USB driver.

The disadvantage of such a setup with multiple recording interface is, that you can't route across the interfaces.
It is not possible to route HW inputs and SW playbacks of interface#1 to HW outputs of interface #2 with near-zero latency.
A workaround is to use e.g., ADAT to route up to 8 ch in single speed between the two different interfaces, but this spoils your ADAT ports and is also from operational perspective not so good to operate. You have to work and route between two TM FX instances. You can of course route through the DAW … but then you have always the full round trip latency in the communication flow. Depending on the ASIO buffer size that you have to use in your project this can add much latency.

A better solution is a setup with only one recording interface and one TM FX instance where you can route any HW input and SW playback channel to any HW output.
But to also get DC coupled outputs you would need to get the new UFX III, as there are not so many RME interfaces which have this option for all analog channels (not only headphones), currently only UCX II, 802 FS and UFX III.
Details see my Excel comparison sheet here: https://forum.rme-audio.de/viewtopic.php?id=35156

As I am not sure what your top priority is, I give you below a few suggestions, how you could achieve more analog channels by using only one recording interface, so that routing through DAW can be excluded completely.

Not sure whether PCIe can be used for your setup, but the RayDAT (a pure digital card) offers 4x ADAT I/O and gives you much flexibility in terms of different types of I/O. In this setup no DC coupled outputs possible as also the Pulse 16 doesn't offer those.

The new UFX III would offer DC coupled outputs to you. But in terms of more analog channels you would need to add a converter like the Ferrofish Pulse 16 MX, to be able to connect it through MADI. Because MADI offers 64ch in single and 32ch in double speed, then you can use this setup for single and double speed (44.1/48 and 88.2/96 kHz).

Note: I left out the 802 FS below, for two reasons, not 100% whether you see it the same way:
- the two ADAT outputs would limit the 16ch AD/DA converter (Pulse 16) to be operated in single speed or you would loose 50% of ports in double speed
- the mic channels are not digitally controlled (I personally do not like this), so you can't store a complete setup including mic gain, phantom power, ... in snapshots

Note 2: I left out the UFX II for two reasons
- has no DC coupled outputs
- has only 2 ADAT ports, this limits you to single speed, as mentioned above

1) RayDAT + Ferro Pulse 16 = €599+€999 = €1598

PC---USB---ARC USB (optional)
| PCIe
RayDAT<-----ADAT 1-4------->Ferro

or

2) RayDAT + Ferro Pulse 16 + ADI-2 Pro FS R BE =  €599+€999+€1699 = €3297

PC---USB---ARC USB (optional)
| PCIe
RayDAT<-----ADAT 1-4------->Ferro
         \AES
          \-------------------------->ADI-2 Pro-------monitor/phones

or

3.1) HDSPe MADI FX + Ferro Pulse 16MX                          = €1439+€1249 = €2688
3.2) HDSPe MADI FX + Ferro Pulse 16MX + ADI-2 Pro FS R BE = €1439+€1249+€1699 = €4387

PC---USB---ARC USB (optional)
| PCIe
HDSPe MADI FX-----MADI1--->Ferro
         \AES
          \-------------------------->ADI-2 Pro (optional 3.2)

or

4) UFX III  + Ferro Pulse 16MX = €2689+€1249 = €3938

PC---USB---ARC USB (optional)
| USB3
UFX III-----------MADI----------Ferro

If you require DC coupled outputs, I think this is not offered by UFX II yet.
And with UFX II (with only 2x ADAT I/O) you would be limited to single speed when getting Pulse 16.

Therefore, I would recommend as of now to get UFX III and Pulse 16 MX via MADI (option 4).
You can sell the UCX to get some good money to cross finance.
The UFX III has the same converter as the former ADI-2 Pro FS.

If single speed and the number of channels is enough maybe this option:

5) UFX III  + old UCX in stand-alone  = €2689

         PC------------USB---------ARC USB (optional)
USB3 |  | USB2
         |  +--------------------------------------------+ (to operate the UCX and for firmware upgrades)
         |                                                           |
       UFX III<----------------ADAT--------------->UCX
                      route up to 8ch from/to UCX

This solution is limited to single speed otherwise you would in double speed only have 4ch via ADAT
Replace UCX with Pulse 16 MX later and connect through MADI.
This way you can split the investment.