One way to do it, former user, is to choose figures for the campaign you are most interested in, and don't get hung up about it if they aren't quite right for other campaigns you plan to play. Lots of people do that.
I paint slowly and infrequently, so I empathize with you, and understand the appeal of a single one-size-fits-all army over having to paint lots of different ones. But I enjoy painting too, so it isn't too a great burden for me.
Find an army that's attractive, add in extra bits that they don't have from other armies and play with them anywhere suitable.
We have a large 15mm Zulu army, bought because we liked the figures. It has only fought an 1879 British column once. But it has occasionally fought against arab slavers, alongside allied arab slavers against Napoleonic British, against Napoleonic French, and with archers added, as a Nubian army against ancient Egyptians, but most frequent of all against 16th century Portugese. We have no problems with it. We know the figures are Zulus, but for other settings, they aren't Zulus they just look like them.