GXL:BL1:9ac50e:a1974b;FEAT:ADFC31AC;POC:3;RCY:0;EMMC:0;READ:0;0.0;CHK:0;
ERROR! Customer ID not match!
The first line is what I would normally expect to see on a GXL (S905X) board indicating BL1 (silicon) cannot find BL2 (software) to boot the board. The second line i've never seen before and my first thought given the wording, is that BL1 is looking for a locked BL2, e.g. in addition to the normal FIP creation process, the boot firmware has been encrypted (signed) with a specific customer key.
The eMMC is blank though, so you can experiment with e.g. VIM1, VIM2, LaFrite, LePotato, Radxa Zero 'board' images written to an SD card to see what happens on the UART connection. Create the SD card then change the dtb filename in extlinux.conf to meson-gxl-s905x-p212.dtb and see what happens. If any of them run u-boot without experiencing a fail/reset/reboot cycle, you got lucky and the FIP sources for that board will work with your box too. If you see the Customer ID error again BL1 is locked and the only firmware that will boot the board is the original Android ROM that's been signed with the correct key. I have a hunch it's the latter scenario, but the only way to find out is trying.