TTL Logic for displaying large 7-segment values
For questions like this it is often good to look at the architectures of early computers.

You (seem to) want two things:


calculations on values
display values (n decimal)


You can optimzie of either:


use binary for easier calculations
use BCD for easier displaying (in decimal)


Both routes have been taken. I think in retrospect in most cases the first is to be preferred.

If the amount of logic required for either calcualtions or conversion to digital format (which involves calculation, espceially division by 10), thing serially: a one-bit ALU requires only a small amount of logic (but it is , of course, slow). This route was also taken in some earlier computers.