So… screwed up, as:
- I followed the steps in [HOWTO] Upgrade manually from e-Nougat | e-Oero to official e-Pie builds without data loss, which include swiping quite everything except data.
- then tried installing latest q build for my S9+ from TWRP, which failed because of error 7 (same as Randy’s error)
- then… phone has no OS anymore…
- then I downloaded a stock image from sammobile (long process, but works)
- then installed heimdall on my Ubuntu box
- then unzipped / untared / lz4 the whole thing (see https://ar.al/2018/07/15/flashing-stock-firmware-onto-a-samsung-galaxy-s9+-sm-g965f-on-ubuntu-18.04-using-heimdall/)
Now I have a bunch of .bin ans .img file, BUT I don’t want to flash blindly according to the PIT output, as I want to keep TWRP installed.
Is this even possible ?
What partitions should I flash to “update the Android firmware” to Q, and then be able to install latest /e/ q image for S9+ (without bricking my phone if possible 
)