A year or so ago I screwed around with an older/smaller Onn Surf 8 (100071485), the punchline is ignore the awful android hacker crapware and use the open source MTKClient to talk to it. I haven’t wholesale flashed it with a ROM or anything, but I have rooted and de-crapped and such.

There is a sad little subforum on xda for them, which at least is enough to see that it looks like the Gen3 ones are still Mediatek parts and the same approach should work.