Okay, I have received my 1.8V UART and can now try and experiment, so this is just what I have had, trying to reproduce what you have done.
Re: something goes bad and keeps on rebooting.
I think the problem will be the root=/dev/ram (I think that was for me at least), but to be sure, look for 'panic' in the command line output. E.g. try
stty -F /dev/ttyUSB0 speed 921600 raw
grep --text panic < /dev/ttyUSB0
I expect you would get something like
Unable to mount root fs on unknown-block
to appear in your output, as /dev/ram probably won't contain a mountable filesystem.
If you have trouble getting to fastboot as it just keeps rebooting, try using xflash, something like:
./xflash enter-fastboot Path-to-mediatek-x20-aosp-16.10/Images/Normal\ Image/MT6797_Android_scatter.txt
you can get xflash from http://builds.96boards.org/releases/helio-x20/mediatek/aosp/latest/