Thanks for the hint.
Restoring that mac80211 patch restores wifi functionality to R7800 with r3845.
To summarise findings so far:
- booting is prevented by
r3820 d5b10bb560 :
ipq806x: make the dwc3 driver and required phy drivers built-in - wifi has been broken by the removal of 936-ath10k_skip_otp_check.patch by commit https://git.lede-project.org/?p=source.git;a=commit;h=cc189c0b7fa015978b04bb663a75b1da726376b5
I reverted the dwc3 commit and restored the 936-ath10k... patch and now R7800 boots again and has wifi.