SpaceGoblin,
Sounds like a very original project. I have another company to suggest, especially after seeing your c.1918-1939 armored car: Empress Miniatures.
They have an excellent and wide-ranging 28mm Anglo-Zulu War range that can fit with your Perry Infantry, and also a "JAZZ AGE IMPERIALISM" range you may find particularly useful, with relatively rare troop types and a wide range of "Interwar Vehicles", such as this Model T Ford:

Here's a LINK to their site:
https://www.empressminiatures.comPS. They also have a Spanish Civil War range that includes more 1930s tanks & armored vehicles, and a range for the 1935-36 Italo-Ethiopian War, both of which might have some useful items for your project.