Bad news/good news:
The shop that was going to do the upgrade to Marshmallow for me was not able to do it. The stock build is no longer available in Sony’s archives, thus they couldn’t use Emma (via Windows) and weren’t experienced with newflasher either, so couldn’t figure out how it works.
But then I learned that pre-Marshmallow Sonys need xflasher instead of newflasher, so I took the device home, found a stock ROM from a developer site, and, using Linux Mint, flashed Marshmallow. It took some trial and error, but I figured it out.
Good news/bad news:
So now I’m finally on stock Android 6.0.1.
But after flashing the boot.img and the PBRP image, I couldn’t boot into recovery, so couldn’t install the custom ROM. Besides that, the tablet wouldn’t even power on, so I had to flash stock again for now. I’ve gone through the process several times, but no luck.
Does anybody have any suggestions?
[Marking this as Solved, since I was able to upgrade stock Android, even if I’m struggling with the custom ROM for now.]