Against best advice, I took the risk to power up the board in its present state.
The power LED flashing 6 times means that the Kickstart's diag routines failed somewhere, and a red screen signals a ROM checksum failure:
https://www.amigalove.com/viewtopic.php?t=324
This is actually great news: the 68030 was able to execute some ROM code and even write into some of Denise's registers.
The checksum error could also be caused by a fault on either the data bus or the address bus.