Sorry for the silence - my tablet finally arrived (I got the 3 GB/32 GB version). I let it update to Android 9 and flashed Ubuntu Touch successfully - works like a charm!

I had to fiddle around a bit to get it to work - nothing big, mostly mashing buttons to make it reboot properly. I did notice those things though:

• When installing, one fastboot command failed and the error message (reported as JSON) said a different command needed to be run to finish unlocking the bootloader. Running that command manually and retrying worked. I don't have the command as I'm currently not next to the computer I used to flash, but I noted the command somewhere and I'll post it here whenever I can.
• When shutting down my tablet while leaving it plugged in, it can't "boot" to the battery charging screen; it stays stuck to the "Lenovo - powered by android" screen forever, which means it stays on all of the time it's plugged in for charging.
• This might be a problem with Ubuntu Touch rather than this port, but when I go to the lock screen, I can hide the side panel, but when I try to type in my password, the keyboard has a slight offset to the right, large enough to contain the side bar, even though I hid it.

I also have the classic issues (can't use adb and so on) but for the rest, it works amazingly well!

If the devs are around, I remain available for testing, so if there's something useful I could do to help, please let me know.

I returned to the website and tried a bunch of images again - I noticed that this time, SP Flash Tool didn't complain about the image-provided DAs, so I used those instead of the default one.

Although all are Android 10, I noticed slight differences between each version, such as whether the installer's "return" mechanism was done with an edge swipe or with an arrow at the bottom of the screen. I took care to check the build ID to make sure the image installed properly.

Here are my findings:

• TB-X606F_S300554_220303_BMP -> Android 10, arrow (default Lenovo rescue image)
• TB-X606F_USR_S100168_200828_BMP -> Won't even boot (used the image's DA, though I also tried using the default DA and it wouldn't boot either, though the SP Flash Tool could re-flash it in both cases)
• TB-X606F_S300007_200923_BMP -> Android 10, swiping
• TB-X606F_S300091_201215_BMP -> Android 10, arrow
• TB-X606F_S300130_210122_BMP -> Android 10, arrow
• TB-X606F_S300177_210304_BMP -> Android 10, arrow

At this point I assumed any S300XXX and subsequent numbers were not what I was looking for, so I tried the only two smaller versions:

• TB-X606F_S000009_210526_BMP -> Android 10, arrow (Some weird "Lovely B tv" boot logo, and some related apps on the home screen)
• TB-X606F_S100015_191127_BMP -> Won't even boot, part 2

At this point I abandoned; I used the Lenovo rescue and Smart Assistant tool to flash a working image and I'll try to bring my tablet to some repair shop to see if they can help.