In my search for One Plus 7 Pro Oxygen Firmware version 9-pie (lower android 10) I always end up on XDA.However, I cannot judge whether these versions are country-specific, i.e. suitable for your place of residence.
Since StockROM v 9-pie works, I would try another custom ROM, e.g. LineageOS 16.0. Also try with microG. If it doesn’t work like under StockROM, the error is probably due to LOS 16.0, less to microG. It’s worth a try, right?