Okay, I stumbled upon something really, really frustrating!

I couldn't flash the boot partition (fastboot: error: Couldn't parse partition size '0x'.) and finally found this Reddit post, which hinted at downloading an older version of fastboot.

I am running Manjaro, which provides android-tools version 30.0.5, and that isn't compatible with my tablet - I guess the "dialect" it speaks with the device is too new, or the device is too old and not supported anymore.

I then downloaded version 29.0.5 from here and with that I was able to "fastboot flash boot boot.img" as expected.

I hope this helps someone else banging their head at the wall...