Nothing but good news today:
The new relay board arrived and it's working perfectly:

While soldering it in, I discovered a cold solder joint in the mute-while-switching-block. Fixed it, now there is only a little, totally acceptable pop noise when switching EVEN IN SPILLOVER MODE in loops 7 and 8.
So, it's almost done. I would love to add a mute function for tuning to one of the loop buttons (activated when held down, deactivated with another press). Also have to write the documentation, which will not only be a build guide, but also a user guide.
For those keen on boards, I can promise you, that the boards (a set of 6 boards for one looper) will be up for sale latest on 20th of April. I will let you know in this thread.