Just as a confirmation to future readers who are considering process: I went through it on an IGG Omnia (which is probably the earliest commercially available model). I installed turris-nor-update, reset the U-Boot environment to its default and followed through @drhm’s excellent guide otherwise – I found this to be the most complete and up-to-date resource. Adapting the schnapps config wasn’t required (as pointed out by drhm).
@tmoz, in case you aren’t able to extract a working snapshot from your system you can as well go with the “naked” medkit archive from CZ.NIC and follow the guide from there. That would equal a factory reset.