EDIT: Rephrased

There are several options.

1. If you want Thunderbolt, some considerations:

Then the UFX+ would be the only option, because RME chooses the digital interface towards the computer always according to the real bandwidth demand.

But please consider the following: RME is able to get 68 I/O (68 input and output channels !) through USB2, you will only find Thunderbolt (and USB3) on the flagship interface UFX+ because it has a very high I/O count of 94 (the usual 30 like UFX/UFX II, plus 64 on top by MADI)-

Other vendors of recording interfaces use nowadays Thunderbolt mainly because they only get more stability for the setup or to reach low RTL times (Round Trip Latency, between recording interface and computer). But by this they increase cost on the recording interface and PC side. Furthermore some of these interfaces do only offer Thunderbolt (i.e. Focusrite), then you can not connect them anymor to other devices via USB, if there should be such a demand.

This all is not necessary for RME, because they have stable USB drivers with very low RTL. By this there is for RME no requirement to implement thunderbolt  in cases where it is really not required and would only raise the manufacturing costs or the technical requirements for the computer. Remember, not every computer nowadays has thunderbolt.

Here a tabular, about RTL times of different RME interfaces, you will see that the times between USB, PCIe, Thunderbolt are very close to each other:

And when comparing RTL times please look at the DAW, what RTL times for input and output the ASIO driver reports.
You can not simply compare two devices by comparing only the lowest amount of ASIO buffers, that are supported.
When looking at some internet reviews you will see, that RME has usually lower RTL times at any given ASIO buffer size compared to the competition. It's not only the ASIO buffersize, it's the quality and efficiency of the ASIO driver, which RTL times you will finally get.

2. You could upgrade from the UC i.e. to an UFX+, this flagship interface would give to you many advantages

- thunderbolt and USB3
- advanced analog section compared to the UC and even to the old flagship interface UFX
- Autoset, which automatically reduces the gain if required to prevent distortion at over 0dB, also useful for leveling
- analog channels support up to 3 difference reference levels per port
- very nice phones outputs supporting two different output levels to be able to even drive phones with higher impedance
- DURec as standalone or backup recording (on top of DAW recording for more safety) to USB disks/SSDs or pen devices
- MADI, up to 64 channels over fiber (still 32 channels @88.2/96 kHz) and distances up to 2km between devices
- all this on a a very compact formfactor (only 1 RU)

The UFX+ is an extremely flexible and versatile unit, very scalable by MADI, see my blog articles here:
About the UFX+/UFX II: https://www.tonstudio-forum.de/blog/ind … 8-RME-UFX/
About UFX+ extensions via MADI and it's setup: https://www.tonstudio-forum.de/blog/ind … Cber-MADI/

3. A third alernative is to enhance either your old UC (or even the UFX+)

The ADI-2 Pro FS use excellent AD/DA converters from AKM, which brings excellent AD/DA conversion quality
and a lot of useful features to a very fair prince considering all the features of this unit.

The ADI-2 Pro FS would enhance both for you, D/A quality for your phones and also for your main monitors.
Plus extremely nice features like dynamic loudness, automatic reference level selection,
slow rampup of volume if you plug your phones, 5- band PEQ, support for balanced phones, etc...

The ADI-2 Pro FS can easily be integrated into any existing setup, you only need a digital output on your recording interface, be it AES, ADAT, optical SPDIF or coax. SPDIF.

If the ADI-2 Pro FS should be too high in terms of price tag, there is another possibility to get the ADI-2 DAC, which is more for HiF use, but of the same quality. It simply lacks some features, that are mostily required in a studio.
- DAC has no A/D, no AES
- ADI-2 Pro FS has two equal phones outputs, also supporting balanced phones, DAC has phones output and IEM output
- Only DAC has remote control

I wrote a blog article that shows how easy it is to integrate the ADI-2 * into your setup. Very useful is the possibility to customize the keys of the front, which gives you a nice user experience, to be able to i.e. quickly choose between main out and phones (with a nice rampup of volume) and other things: https://www.tonstudio-forum.de/blog/ind … our-Setup/

Especially about the DAC and how you can nicely integrate it also near your HiFi equipment, read this blog article: https://www.tonstudio-forum.de/blog/ind … her-EN-DE/

This article shows some of my use cases for the ADI-2 Pro: https://www.tonstudio-forum.de/blog/ind … C3%A4rker/

4. In regards to the SSL SIX

If you have a device with TotalMix FX, then I think in most cases you do not require a separate mixer.
And yes it has no digital ports. Not so useful IMHO.
On a recording interface inputs and outputs are usually of higher quality anyway: SNR, etc.

5. In regards to the Antelope

> Antelope sounded waaay better but worked like shit (latency, stability etc.) so I had to return it.

That's the advantage of RME to produce well balanced products, where also driver and software are very good.

5. Summary

So I would stay with a compact solution and keep the UC:
- UC + ADI-2 Pro FS
or if you want a but more (Durec, Autoset, even better analog section compared to former flagship):
- UFX+ alone, which enhances analog I/O already compared to UC
or a combination of an advanved recording interface and the ADI-2 Pro FS:
- UFX II and ADI-2 Pro FS (USB2)
- UFX+  and ADI-2 Pro FS (Thunderbolt, USB3, USB2)

I tried to give this overview as comprehend as possible, but some details I also wanted to mention.

Shall there be something unclear to you, please feel free to ask.

I am personally very satisfied with my RME gear and I think there is currently nothing better available for the price:
1. for the studio a combination of UFX+, Octamic XTC via MADI and ADI-2 Pro FS for monitors and phones
2. for HiF I use the PC as player, all D/A conversions are nicely done by an ADI-2 DAC in front of a high-end HiFi.