Magisk

• Install magisk.apk from Magisk releases
• Send boot_a.img and boot_b.img to the device

adb push boot_a.bin /storage/emulated/0/Download
adb push boot_b.bin /storage/emulated/0/Download

• In Magisk app click to the install button
• Choose method Select and Patch a File
• Choose boot_a.img from Download folder
• Click to the LET'S GO -> button
• Get patched images to computer

change random strings with yours

adb pull /sdcard/Download/magisk_patched-[random_strings].img patched_boot_a.img

• Reapeat that with boot_b.img
• reboot to bootloader

adb reboot bootloader

you can try to boot device with fastboot boot patched_boot_b.img if it booted normaly you can flash it.

• flash kernel

fastboot flash boot_b patched_boot_b.img
fastboot flash boot_a patched_boot_a.img 

• Then reboot

fastboot reboot

• Done 😃