YouTube, Netflix & Co started to stream AV1 encoded content and so on
Yes they started to stream in AV1, but until they stop streaming in H264, VP9, etc. and ONLY stream in AV1 the lack of support for AV1 in boxes isn't really an issue. Hardware will catch-up eventually, but there's no rush.
If you think this is a Kodi bug you need to go file it with Kodi devs in the Kodi forums. It's not an LE bug.
How about you go install it on an SD card and have a look?
I have a Vorke Z6 with a S912 and 3G ram. Ive tried both of the gxm device trees but cant get it to boot.
Both? .. there are at least 8x device-trees for GXM boxes and they are all mostly identical. If none of those work .. 
unless you attach a UART cable and share the u-boot serial console output so we can see where it gets stuck. Note the device-tree process is different in mainline images: You need to use the "box" image and edit the dtb name in uEnv.ini. if you start renamng files like legacy images do that's one reason none will boot.
Now I’m a bit lost and would like to ask, what box / SoC with AV1 HW support should I look at, which is supported by LE? Any new recommendations one year since the last post here?
AV1 requires the latest Intel chips, which are still a little experimental in places, but you know Intel is committed and will push all the driver support upstream within a reasonable amount of time. None of the SBCs have upstream support at this stage. FWIW, I've always regarded Ugoos as one of the cheaper and more awkward vendors - but that's probably because they refuse to share sources and schematics for the boxes (I'm biased).
Pics show a random circuit board with an assortment of standard connectors; it could be anything. The sole useful bit of info is on the chip under the heatsink. Unless you share that I can only make blind guesses, and I guess your box is not supported.
The name means nothing and "rxj1909-MB-v06" returns zero results in Google so we have no clue (we are not omniprescient or mind-readers). You will probably need to power it on and see what Android claims it is. Worst case you will need to open up the box and take some close-up and in-focus pics of the board with the chip exposed. If any of that seems like too much effort, you should probably stick to Android.
I'd recommend Musicbrainz Picard for organising audio files: https://picard.musicbrainz.org/ as it will also embed their IDs in the files which guarantees things scrape well. It's also the app used by the folks who dev on Audio topics in Team Kodi. Reworking a large library with Picard might take time, but it's one of those tasks that you only need to do properly once, and then future re-scrapes (should they ever be needed) become simple.
If you really want to do things manually.. have a read https://kodi.wiki/view/Scraping_Music
U-boot can boot from an EXT4 boot partition; assuming the embedded bootscript that searches for boot files (extlinux.conf, or boot.ini, etc.) has been adapted to it. LE scripts assume FAT, so you things like "fatload" in them.
SSH into the box and run the commands "dmesg | paste" and "fw_printenv | paste" then share the URLs generated.
Yes, it's broken for a while now (and nobody understands Qt properly to fix it). Please use Etcher to create USB/SD cards.
U-boot also allows the entire boot process to be encrypted/signed and protected; but this requires support for those things in Silicon which increases costs, so you almost never see it with cheap TV box devices.
It looks like "quiet" is missing from boot params (so you see the earlycon boot content).
RPi4 can boot via u-boot, but it's not needed and we don't use it with LE images for RPi devices. And if the user needs to ask Q's at the current level the likelihood of them building a custom image that implements u-boot support for RPi in LE is rather unlikely IMHO.
liberi That isn't going to work on an RPi4 which doesn't use Grub2 (or any other normal bootloader).
Take a backup of the .kodi folder and contents from the RPiOS install, store it somewhere safe. Once you installed LE via PINN or NOOBS for some level of multi-boot support you can copy the backup file over and manually restore content by stopping Kodi, moving files to the right places, restarting Kodi again.
NB: It is not possible to install LE into a partition. It is not possible to use an NTFS drive as the /storage partition. You can use the NTFS drive as a separate media drive that's shared between several OS without issues, but /storage needs Unix filesystem permissions support.
I know nothing about RK hardware
Older devices may also need the "Use Legacy Security" option enabled.
