Shimano's upcoming gravel Di2 is meant to be using micro spline hubs (to get 10T), so that might be a better option for those wanting 10T with ERX (use micro spline wheels and cassettes), as the chain spacing should be the same as standard Shimano HG (I believe XDR chains have slightly bigger gaps)
There are people on youtube that are using Shimano cassettes with AXS and the other way round. So I would have thought ERX would technically work with XDR cassettes, but there have been reports that the shifting isn't quite as good when mixing AXS with Di2 (12 speed).
I was just watching a Eurobike video on Youtube (forget which. Peak Torque??) but the LTwoo stand has a bike with ERX on display for testing and it's set up with Force AXS crankset and cassette.
Shifting on my AXS setup with Force cassette is hardly better than with R7100 and R8100 cassettes. I notice the chain is more stable when back pedaling with the AXS chain and cassette, but difference in shifting performance while riding is negligible when compared to Shimano cassettes and KMC chain.
Front shifting was also pretty solid with Praxis chainrings no matter what chain I was using, again no worse than a full AXS setup. YMMV of course, but in my experience the whole Shimano and SRAM 12 speed incompatibility is heavily overblown. Not saying everyone should start mixing components and stop worrying about component wear immediately, but IME it's no big deal. The F/R derailleur setup is the most important part. Use what you want and set it up well, it will work. I even did a few miles on a cheap $45 Aliexpress 12 speed cassette and shifting was pretty damn good.
