If you want to store obb files to external storage, there’s only one viable option that I found.

It’s fbind magisk module originally created by @VR25 and updated by @Hooin Kyoma. Here’s link to updated fbind module.

To use this, install termux terminal app. Type “su”. Approve root access. Type “fbind”. Type “3”.
Type “1” to move everything from Android/obb to external
Type “2” to move specific folder and then type the name of the folder you want to move.

It’s also advisable to have SD card that has “A1” or “A2” app class for better IO performance.

https://forum.xda-developers.com/t/rom-lineageos-18-1-unofficial-2-0-update-2021-01-20.4047763/page-29