I've had really good experiences with USB over the years, so I disagree with your idea that USB3 alone would not be well enough for an interface like the UFX III.
Back in 2014, I started using a UFX with USB2 and FW400, but USB ended up working better than Firewire.
Then, in 2015, I got a RayDAT and tested whether a PCIe card would give me any advantages in terms of CPU load or latency. I used the UFX in standalone mode, connected through ADAT, and found that it gave me no visible benefit over USB2 in terms of CPU/system load or stability. So, in my experience, USB2 worked just as well as a PCIe-based RayDAT.
I upgraded to the UFX+ in 2016 and found that using it via USB3 worked great, even with its high channel count of 188. And for even more channels, the MADIface XTC has 394 channels and is also well-supported by USB3.
In 2017, I wrote a blog article comparing UFX+ via USB3 and RayDAT via PCIe under load. I used a big Cubase project with 400 audio tracks, 2 VSTs per track, and 3 in the sum, so 803 VSTs in total. Even at the lowest ASIO buffersize of 32 samples at 44.1 kHz (or 64 samples at 96 kHz), there were no audio drops during playback, and the CPU and ASIO load was handled very well by my system (meanwhile a 7-year-old CPU). The conclusion was that there was no visible difference between using UFX+ USB3 vs. RayDAT PCIe based. Thunderbolt is "external PCIe" so it is compareable.
BTW, I just uploaded a new screenshot where you can see this test project with 400 tracks and 803 VST running on my current system with last recent Cubase 12.0.60 Pro:
Latency-wise, the difference between USB3 and Thunderbolt is tiny. I have a comparison chart of different solutions that I own(ed) that you might find helpful, see here: https://www.tonstudio-forum.de/attachme … es-v2-jpg/
If you're interested in getting some more information, like e.g. differences between UFX+ and UFX III and some recommendations, then please have a look at this thread:
https://forum.rme-audio.de/viewtopic.ph … 62#p200962
I would encourage you to try USB2/3 with the UFX series for yourself instead of relying on hearsay or unproven expectations. I also recommend getting a USB3 PCIe card with a supported USB3 chip, like the Sonnet card with FL1100 USB3 controller, to isolate your recording interface from the rest of your USB infrastructure. This can proactively prevent the addition of other USB-based devices from negatively impacting your connected recording interface.
One proactive tip, like I recommended in the above-mentioned thread (again the URL: https://forum.rme-audio.de/viewtopic.ph … 62#p200962). I would directly get a USB3 PCIe card with supported USB3 chip like the Sonnet card with FL1100 USB3 controller to fully isolate the recording interface from the rest of your USB infrastructure. It can proactively help, that adding more and more USB based devices (headsets, Bluetooth adapter, etc.) have no bad impact on the connected recording interface. For example, this product: https://www.sonnettech.com/product/alle … 4port.html
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13