i don't see any need or chance for APS-C sensors in Canon EOS R line. The sole reason, why crop sensored imaging products exist are size and price. Both objectives are perfectly met with EOS M/EF-M.
R-mount is perfect for FF and will be used exclusively for FF, simply because R-mount bodies with crop sensor and lenses for it could never be made as compact as EF-M gear. Canon people in charge have confirmed this in a recent interview. APS-C in EOS R line does not make sense: neither techically, nor commercially.
compact and affordable = EOS M, EF-M plus option to use all EF and EF-S glass. original Canon EF/EF-M adapter works perfectly well, no issues whatsoever (i have it, i use it). limited AF performance is an issue only with some older EF lenses that were not designed for use in live view/mirrorless mode.
With launch of M5/M6 and especially M50 and subsequent EF-M lenses (28 macro, 18-150, 32/1.4) and undoubtedly more to come (like a fisheye or next M bodies) Canon have finally accelerated transition from crop-sensor mirrorslappers (xxxxD, xxxD, xxD, 7D - in that sequence) and EF-S mount to mirrorfree M/EF-M.
there is no reasonable doubt that EOS M line will be Canon's crop-sensor platform and in terms of unit sales their most important product line for many years to come.
btw: "no upgrade path" In form of "buying and using FF lenses on crop sensor cameras" is not important any longer in reality.