Fun project, sharp.
Especially the scythed chariot.
https://www.leonardodavinci.net/scythed-chariot.jspOr the machine gun crossbow.
Looting a big water mill model comes to mind.
https://www.leonardodavinci.net/machine-gun.jspFor the enormous crossbow, you could just tinker with a real one.
https://www.leonardodavinci.net/design-for-an-enormous-crossbow.jspBy the way, the da Vinci tank would have been quite big.
It was to be manned by eight men, four cranks, three loaders, one commander.
The diameter would have been about nine meters.
Whatever scale 28mm maybe for you, but for simple math let's take it 1/50.
That makes a diameter of 18 cm for the model.
Since that monster was intended more as a psychological horror show than a tactical kinetic weapon, I'd say the bigger the better.
Especially if you want to build the interior.
You should add an aerial screw to its top.
Just for a good measure and to cross rough terrain.