I wonder if it would be possible to replace the War Wagon with another unit for Germans civ - Mounted Infantry (from Habsburg Royal House). In this way, a completely new Minor Civilization could be created that would specialize in sieges - Hussite Camp (visually it could resemble Karlštejn Castle surrounded by siege machines and Chapel).
Hussite Camp could offer training in War Wagons, Trebuchets and firearms precursors - Hákovnice (Unfortunately, it’s not in English).