For ease and economy of implementation, the BCD decoder is made of a diode matrix, as described at https://hackaday.io/project/8121-discrete-yasep/log/26997-redneck-disintegrated-7-segments-decoder

The modified/truncated table is shown here:

There are only 21 diodes to wire thanks to inverted logic.

A detail that has not struck me immediately is the absence of diodes for the number 8 (though I've already implemented this simplification in the hexadecimal decoder's diode matrix). No pull-down means no gate: this spares 3 AND2 gates, or 6 transistors from the outputs of the Johnson counters that deal with the units (0-9).

That's something.

Wait.

The number 2 is the only one that turns segment C off. This means that, at one point, a diode is not necessary. There will be 6 digits that pull this wire down (the 6 Johnson counters with their paralleled AND2s) but it's the only affected segment. We're now down to 20 diodes.

How can I save even more when only one segment is turned off ? Numbers 0, 6 and 9 might be the next candidates.

It seems it's possible and only 17 diodes remain... Here is the schematic (only one output MOSFET is shown) :