I hope the R7 will be the R version of the non-existent 7DIII.
While I have no idea how Canon choose their designations I'd have expected the 90D to become an R10 and the XXX range an R100 so that gives a similar upgrade path to the EF ranges.
However, wanting to move to FF (I was all ready to get a 5DIV) the R6/R5 route seems possible.
But with an R6 I would need a new UWA lens as the crop mode is lo res , and probably the longer lenses too as the pixels are larger. Maybe stretch to a 100-500.
With an R5 I might just be able to keep the 10-22 in crop mode and save a bit initially. But still need the 100-500.
So an R7 would be ideal starter for me if it had, say, 24MP BIS for low noise. Oh, and eye focus selection as a BIF unit.