I nailed it !
I solved a "bug" and I now use "normal" hexadecimal encoding knobs. This required a big redesign... There are now 29 diodes but only one per signal so I can use old, low-current point-contact D9K.
Another big difference is a rotary 4-poles 3-throw selector ! It selects between the Imm8, Imm4 and Register forms. I could use interlocked switches as well but their mechanical installation might be more complex, more holes and alignment...
I'm only missing one row of 8 switches for now, but I can start the construction :-)