I noticed only now some details I didn't see before: these 3V relays are rated for 150V 100mA max. (under which conditions ?)
Given that it takes 60mA to energise one coil, the amplification coefficient is barely 2 and the design expects more.
It was a good idea to connect coils in series wherever possible. Control signals still require a large fanout and I'll probably have to implement a 2-levels signal tree (with repeaters for each nibble or two).
Given the current handling, it would have been wiser to opt for a higher coil voltage, which reduces the required current, thus a better amplification factor.
That, and DPDT, will be considerations for a next version, if any !