[Oukitel (All phone models)] MarkR7:
OK, it is possible to install Android 11 GSI system images on this phone. Android 12 images do not work, something to do with incompatible boot partition.

It's possible to try out GSI system images without flashing by installing this app:

GitHub - VegaBobo/DSU-Sideloader: A simple app made to help users easily install GSIs via DSU's Android feature.

A simple app made to help users easily install GSIs via DSU's Android feature. - VegaBobo/DSU-Sideloader

github.com

Root is useful, but not necessary. Seems not to work, maybe needs flashed empty vbmeta_system and vbmeta_vendor like indicated below and I restored all to stock ROM except for boot.img with Magisk 24.3 and empty vbmeta.img

If you managed to make full backup of ROM with mtkclient, enable USB debugging, unlock bootloader, then to flash GSI images or Lineage 18.1 based on Android 11 (it works, but no image with gapps unless found somewhere else) you have to have installed Google Android USB driver from here

Get the Google USB Driver | Android Studio | Android Developers

The Google USB Driver is required to perform adb debugging on Windows with Google devices.

developer.android.com

Unpack files, connect phone in recovery, power + volume up then power+ volume up fast click, choose fastboot, that gives you FastbootD (colorful menu), which is now used to flash system partitions. Connect phone, install driver with .inf file.

You also need Google platform tools

SDK Platform Tools release notes | Android Studio | Android Developers

Android SDK Platform-Tools is a component for the Android SDK.

developer.android.com

Unpack anywhere if on Windows.

Run cmd, then "fastboot devices" to see if phone gets detected, should be there.

From here get product_gsi.img:

https://xdaforums.com/t/tutorial-magisk-on-gsi-on-devices-with-dynamic-partition.4311045/

In mtk client is an empty vbmeta file - vbmeta.img.empty or similar, needs to be flashed to vbmeta, vbmeta_system, and vbmeta_vendor to disable verification of boot for magisk, and system and vendor for GSI system images. Can be done with mtkclient or normal fastboot ( power + volume down, choose fastboot, connect phone, run cmd, then "fastboot flash vbmeta vbmeta.img.empty (could be renamed to anything simple, name doesn't matter))

Then

fastboot flash vbmeta_system vbmeta_img.empty

Then

fastboot flash vbmeta_vendor vbmeta.img.empty

Same file to all three partitions, disables verification, worked for me. Can be done with GUI in mtkclient, same thing. Or commands in mtkclient

"python w vbmeta vbmeta.img.empty"

Etc.

Back to FastbootD -

"fastboot reboot fastboot" or power + volume up for recovery then click them again, choose fastboot.

Or probably "adb reboot fastboot" if phone is running system.

Run cmd, "fastboot devices" to be sure phone is detected, then

"fastboot flash product product_gsi.img"

This resizes dynamic system partition when flashing system images. This only modifies internal logical partitions within SUPER partition that contains Product, System, and Vendor. SUPER can be restored with mtkclient or just fastboot, I guess, I restored with mtkclient, works.

Then you can flash any system based on Android 11, stock phhusson GSI works. the system needs to be arm64, ab (for A/B partitions, indicated by letter B in name), vndklite, G or Gapps (if you want them), V is vanilla - no gapps, S is superuser but seems to have conflicts with magisk if you want that as root later. N is no superuser/root.

Eg.

"fastboot flash system Lineage-18.1-2021214-UNOFFICIAL-treble_arm64_bvS-vndklite.img" Rename this to 1.img or whatever, simpler to type

"fastboot flash system.1.img" or something like that.

"Invalid Sparse System Image At Magic Header" appears, but doesn't matter, starts flashing in 30 seconds or so, 10 or 11 chunks, maybe 2-3 minutes per system image.

Then

"fastboot reboot" go to recovery, power +volume up, click them again, choose wipe user data, wipe cache to get rid of old settings and apps from previous system.

"fastboot reboot recovery" should work too.

Seems that on custom ROMs Magisk 24.3 app gives option to directly patch boot image without need for flashing patched boot.img. Not sure about that. Just flashing patched boot.img in fastboot gives root, anyway.

If you just want newest Magisk then flash empty vbmeta and then patched boot Magisk generates. Those can be done in normal fastboot. Works fine.

Everything seems to work.