Well, it seems that I've been able to fix it at least partly (and without a serial connection, as I still don't have one). i found an archive (on the CracKdroid Project Chat Telegram group) that was meant for the KT3 and which had a "restore to kindle os" option (combined with booting the android uboot image from
https://github.com/MollySophia/imx6_..._u-boot_kindle multiple times until it got into fastboot mode)
The steps were: (Note: I had to do this in Windows)
1. Booting the Kindle into fastboot mode by using imx_usb_loader to boot the
https://github.com/MollySophia/imx6_..._u-boot_kindle android uboot image 2 times (I changed the "LP_FASTBOOT_THRESHOLD" in
board/freescale/mx6sl_heisenberg/pmic_rohm.c to a higher number so I had enough time to boot the image twice in order for the fastboot sequence to trigger).
2. Used the fastbootdownload.bat file provided by the script in the "kindle8" folder and pressed 3 on the keyboard (the third option means "Restore system (step 1)" as per google translate -- I don't know the language).
3. The script flashed a restore.img file to the bootloader partition and told me to reboot the device using the power button, which I did, and then flashed some other images as well.
4. The script exited and I ran it again and now used the 4th option and the kindle rebooted into a loading state (The kid under the tree had a loading bar) and then I was greeted by the home screen.
The kindle seems to have downloaded some chinese stuff, but I at least can now have more leverage to play with it further.
The archive itself exceeds the maximum limit allowed by the forum, so I will simply provide its filename (it can be found on the CracKdroid telegram group): kindle8.230118.zip