Reading about this over the decades, lions have concluded 4 bits is the smallest viable ALU.  The minimal computer should devote all its components to RAM instead of devoting hundreds of parts to a separate ROM.  Some amount of it should be analog instead of digital.

A discrete component computer is really a study in memory devices.  There's memory for the program, data, registers, & the instruction set.  The biggest win would be simplifying the memory.  Analog memory seemed like the easiest solution.

4 bits could be reduced to a voltage with 16 steps & this could be stored in a single capacitor instead of transistors.  Each nibble could use maybe 2 transistors for addressing.  Some dedicated hardware could constantly scan the memory & refresh the capacitors.

The trick is translating between the digital & the analog parts.

The input & output would be punched cards & LEDs.

At least such a project would be a good way to visualize how computers work at the lowest level.   There's been more interest in discrete component computers because we're moving away from the digital logic of the 20th century & towards the qubit logic of quantum computing.  Quantum computing is starting over at the raw physics & building up everything else from scratch.  It's more like reading analog voltages than digital bits.