Hello @RayMcCoy,
Downgrade form StockROM Android 10 version G960FXXU7DTB4 to Android StockROM Android 9 version G960FXXU7CSJ1 with region code SFR is the only way Samsung allows! You are with the French mobile communications company SFR (Société française du radiotéléphone), right?
A further downgrade from StockROM Android 9 version G960FXXU7CSJ1 is not possible due to Samsung restrictions.
The installation based on stockROM Android 9 to /e/ OS Oreo 8.0.0 is then possible. Recently I accompanied a /e/ user during the installation.
Do it now! The time is cheap …