Thank you, it checked out. I thought I had already posted this, but appearantly not...
Eventually, I got FreeNas installed again, but it was a pain. New USB-sticks (give-aways, but still never used) gave issues: a 1GB with the installation iso on it would not work (which you only find out after ruling out all other problems), 8GB was deemed too small (? I know it (USB2 stick) is a few bytes short compared to another 8GB (USB3) as previously I got an error from FreeNas that it could not use that specific kind of 8GB to mirror the slightly bigger 8GB bootstick onto, but 7.45GB is not enough for a fresh install either?), etc.
I used Etcher, WIN32diskimager and Rufus. I don't know which one eventually did the trick. In the bootmenu, often only one USB would show up. It almost seemed random which option within the USB biosline I should pick (auto, harddisk, FDD). So I could not make heads nor tails from what worked and what didn't but it's installed now.
Both old USB3 sticks are now out of commission (the boot 8GB and the 16GB mirror from the previous install). Even with Active@ Killdisk I could not get them to a state where they could be used as the bootdevice again.
I'm probably buying two 250GB 2.5" drives to replace the USB-boot-stick as I have pretty much lost all confidence in USB-sticks.
Next test is to restore the pool I had before. It's still empty (the six 3TB drives), but still, if I can't restore it, I'm out of my depth here and the solution (a home server with ZFS2 for dataprotection) may be causing more problems than it solves. And I should just hook a USB-harddisk to my wifi-router that I mirror manually once a month for shared file storage.
I did copy the encryption key from the previous install, but not the configuration file.
From within the GUI I cannot find any way to import it ("sixtimes3TB" doesn't show up anywhere), so I'lll read up on the commandline possibilities.