I think you will be very happy with the R6pro II.
I had the Cayin N7 and the Cayin Ru7 donggle, I could hardly tell the difference between the two with Ananda hi-fi headphones. with Monarch Mkii Very slightly.
This surprised me a lot.
But on the more high-end he1000v2 steath or even the multiverse mentor then yes there is more of a difference.
the soundstage especially on the n7 was significantly better.
This r8ii appeals to me a lot now that I'm listening to dap again....and that I sold my n7.
If it could be even better I would be very happy.
If you plan to upgrade later it might be the R8ii.
But you can also start with the r6pro II while waiting to upgrade your iem and buy the second-hand r8ii more cheaply at that time.