To get full control of the phone, I’ve installed LineageOS with root access.

The following instructions might result in data loss, so make sure you have a backup of all data on the phone!

Install TWRP recovery

Download

First, you need to boot into Odin Mode. Press and hold Power+Home+Volume Down.

Confirm you are about to lose warrenty by pressing Volume Up.

On Manjaro, I’ve used heimdall:

heimdall detect --verbose
heimdall flash --RECOVERY twrp-3.4.0-0-herolte.img --no-reboot

Important: Immediately after the screen turns black, press and hold Power+Home+Volume Up to boot into the recovery. If the original ROM is loaded, it will restore the recovery and you need to start over again.

In TWRP

Wipe

Swipe to confirm you want to make changes to the system.

First, format the /data partition. Click on Wipe in the menu:

Install LineageOS

In recovery, you can already connect via adb.

adb push -a lineage-17.1-20201008-UNOFFICIAL-herolte.zip /sdcard/

(I’ve used an image file from xda.)

Reboot

Make sure to uncheck all boxes and click Do Not Install.

LineageOS

Magisk

Now you’ve successfully installed LineageOS, but you don’t have root access (via su) yet. To get it, go here. I’ve used Magisk v21.0 and Magisk Manager v8.0.2.

Install both, reboot, and you should have full control now.