I contacted support, but its still holidays, so no reply yet.
What I wanted to try - without flashing anything yet - was just to override kernel, dtb and kernel commandline for a single boot using a fastboot command.
If you managed to compile the kernel, as above, can you try if you can boot into it at all with something like
fastboot boot arch/arm64/boot/Image.gz --dtb arch/arm64/boot/dts/qcom/sdm632-ext-codec-mtp-s4.dtb
note, NO flash command, it’d be a smart idea not to flash anything to the phone’s partitions before testing it in one-shot mode
this should - if it works - boot the phone up into the default android system, but using the self compiled kernel.
if that works, as a next step I’d try
fastboot boot arch/arm64/boot/Image.gz --dtb arch/arm64/boot/dts/qcom/sdm632-ext-codec-mtp-s4.dtb --cmdline “init=/bin/sh”
to try to just run a shell instead of android.
this likely will result in a blank screen (but if you’re really lucky, the kernel has a working framebuffer console. then you could connect a keyboard via a USB-C OTG cable and browse the file-system)
if there was no framebuffer console, maybe the kernel can be recompiled with one enabled, (make menuconfig) but its possible additional cmdline options need to be supplied to setup the screen correctly
once you have a shell, the next step would be to compile a ramdisk image with busybox or similar and an ssh server, wifi support (wpa-supplicant) etc, to have a working linux to play.
from there its a very small step to a working ubuntu image or similar.