Because custom roms work on top of the OEM android installation it is my opinion that the best base for installing eOS-r onto the phone would be from stock android 11 OOS; though I admit my beliefs are based more on intuition rather than technical know how.
Given that the bulk of the partitions on the phone are stock android (custom rom only flashes over a few partitions) it seems to make sense that to get the best out of the hardware is to keep the stock android up to date for the intended custom rom.
For that reason I’m coming round to the idea that restoring the phone to it’s previous stock android state (OOS 10) and then updating OTA to OOS 11would provide the best base for android 11 custom roms.