Boot loop, think I flashed an outdated boot version

I have an Xperia Z5 Compact with German T-Mobile factory ROM, which is not too much use to me coz I don't live in Germany or use T-Mobile. I wanted to flash a stock ROM with no bloatware. Root probably later, I'd like to use the phone a bit first. It isn't simlocked btw, and was OTA updated to Marshmallow before I tried anything.

So I unlocked the bootloader, then flashed the recovery and boot images from the first post here without noticing they are old versions and further down the thread they are updated. Now I can get into TWRP recovery, but when I try to start the phone normally it freezes on the pink T-Mobile splash screen. Recovering from my Nandroid backup doesn't help... coz I only created it from within TWRP after I did this flash.

I can't use Flashtool, it says USB debugging is turned off. I guess what I flashed before has reset that to off. I tried following the Lollipop instructions here to turn it back on using adb, but I can't pull the file mentioned there. Anyone got instructions for doing the same on Marshmallow please?

Not exactly panicking here, but I really wish I practiced on my old HTC Hero before jumping in blind....

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