How to recover data when everything is locked (FRP, OEM) Working on recovering data from phones newer than Android 8.1. The situation is this:

Customer forgot the lock (pattern/pin/...)
Because lock screen can't be opened, OEM is locked.
Can't install custom recovery (such as cwn, twrp) due to OEM lock.

It seems that the only way is to mount the hard disk which seems to be encrypted by default.
So what should be done in this situation?
data recovery and screen lock by-pass are two different things. both should impossible due security reasons
Android provides official unlock solution via google account. bonded google account is requirement for using Google Play App Store, most likely every costumer will have
google account itself has several forgotten password options, it's up to customer to get it work, not yours
you already have mentioned some workarounds which might have worked on some (older) devices, let me explain a little more
data recovery from factory reset is not possible because of encryption
data recovery from FRP lock is not possible because of factory reset
data recovery via unlocking is not possible because the bootlader unlock triggers factory reset
reset screen lock is not possible because login credentials are partially involved in calculating encryption keys, deleted locksettings.db will destroy encryption
off-chip recovery is not possible because encryption is hardware bound
However, some cracking is possible for some devices under certain conditions
data partition can be decrypted when

device uses FDE full-disk encryption
full-disk encryption uses default_password
decryption is performed on this unique source device itself

screen lock can be deleted when

data partition is decrypted
encryption doesn't rely on login credentials

dump partitions on locked bootloader is possible via

Qualcomm EDL mode when OEM signed factory loader is leaked
Mediatek SP Flash Tool with proper Secure boot Download Agent and authentification files

flashing on locked bootloader is possible when

boot/recovery image is AVBv1 signed