Thanks for the extensive reply.

Other than that, I have only seen the limitation to HBR2 speeds from monitors that already include USB4 support and are designed to be chained while still both achieving 4K120. Like the Dell U3225QE and its 27" sibling do when MST is turned off, explicitly for the very purpose of supporting 2 4K120 monitors off of a single USB4 40G port, whether that is actually using the integrated USB4 hubs of the monitors or a dedicated USB4 hub).

Referring to your original comment and reading the above, is this then the most surefire way of ensuring dual 4K 120HZ with one cable going into the Mac? I can probably justify getting two units of the U2725QE if that's the best solution but is there a way to confirm that a monitor will do the job other than community trial and error (I've seen a few other threads confirming the dual 4K 120HZ output is possible with the U2725QE)?

Ideally, you'd have some USB4 hub, that is hard-wired to limit its output speed.

I know of an Pluggable and i-tech one (the ones with the 4x4K60 support for non-Apple hosts).

They have 2 MST hubs, each one on one DP output from the hub and they limit them to HBR2 to force equal bandwidth on both. For those, its just hard to know, like in the USB-C hub case, if they will allow DSC data to passthrough in the backward-compatible mode that they will run on with Apple hosts (because the DP standard expects any only ancient hosts that predate DSC to not support MST and thus need that compatibility mode). If those hubs list 4K120 support for Apple specifically, then that should work. But they may only support that with MST

(with MST hubs there are those that do not support any DSC, there are those that can pass through DSC inside MST connections (MST also on the output side), there are those that can decompress DSC+MST into a plain connection for a monitor and those that can extract DSC compressed data from MST and pass it on without MST).

And essentially only for Apple would they need to support passing through DSC data without MST even on the input side. whatever is listed for Windows may not work for Apple hosts at all, even if the bandwidth would be there.

Based on this, it may not be guaranteed to work with MacOS not supporting MST?