Your phone is compatible with treble thanks to the community.
You can use vendors we shared in the 7870 telegram device group.
The instruction are very simple : wipe everthing on your internal storage as usual, then flash in the recovery your vendor and the gsi_system.img you want. Use the treble info app in order to know which version you must install. It's arm64 most of the time with CrDroid vendor.
The application gave me this information .. Could you provide me with a link to download "system-arm 64-ab.img.xz" .. Please ..?