Where do I put digital ground in a dual-rail mixed signal circuit?
Of course all grounds (analog and digital) most be electrically connected, as not connecting them will be the road to endless trouble. As Neil_UK suggests, use a star-grounding scheme, see this article. This is to prevent the currents running through the ground connections introducing voltages in the grounds of other (unrelated) circuits.

Minimize switching noise generated by your circuits by keeping the loops short, that means adding supply decoupling capacitors across the supply pins of all individual ICs and especially the logic ICs. Using 2 different size capacitors like 100 nF and 100 pF in parallel is generally a good idea.

The higher the speed of a chip, the faster its transitions will be and the more harmonics it will produce. If you don't need the 74AHC for performance reasons then I'd stick with a slower version like 74HC.

I'd put 10 uF here and there in the supply lines, 10 uF caps do not do so much at high frequencies (above 1 MHz). 100 nF are better so also use those and 1 nF or 100 pF are best for high frequencies so there's usually no harm in adding (a place for) those as well. Note that on a PCB you can just add a place for a capacitor and later decide not to use it. Adding a capacitor in a place where there's no footprint for it is much harder.