Well, I went ahead and installed the stock recovery and this OTA-update.
Everything seemed fine so I re-installed a new copy of TWRP (same version number as I had before)... Confused me totally. It was asking me to root again as soon as I quit recovery, which I did a few times... But I could never finish the installation of SuperSU. It always failed...
I was messing around with this for 2 days, until I for some reason tried to install TWRP from another "old" file I found on my computer (the exact same file I used when i rooted my phone the first time).
Now everything worked fine...
I guess the other recovery image was corrupt, incorrect, or something else...
Well, the mess was sorted out at least, and I still have root, did not have to reset my phone, and everything else seems to be OK.
Thanks! :good: