If you want to have a fighting chance of ending up with a small design, the instruction set follows the control unit and not the other way around. While writing the assembler I found that I had overlooked something in my original 4 or 5 IC design. I didn't want to proceed with anything else until this was settled. So back to the drawing board. I managed to solve everything within 5 chips, but still with a somewhat limited repertoire for conditional jumps: still only BPL and BMI were present. Doable, but not nice. After some puzzling, I figured I could get a complete conditional jump set with just one additional 74153 IC. I think the benefits outweigh the extra IC, so it is in for now. The clock will have to move over to another place on the breadboard, because this will take up the whole upper right board.
Designing the control unit turned out to be a bigger challenge than I had anticipated. I will let it rest for a couple of days. If I don't find any new oversights in that time, this is what it will look like. The instruction set follows from this, more on that another time.
[ Edit: Only one real issue found since writing this: the write instructions had unintended side effects. Also some of the wired logic was wrong, fixed that. We now use the '138 instead of '155, and AC/OUT must become '377 instead of '273. With that it still fits in 6 chips, although the wired AND remains a bit of a wart. ]