Hello,

I am about to start building a Smartrap Core with a build volume of 300x300x300mm.

It will have a : 300x300 heatbed. (if I ever find it in stock )
The heatbed carriage (or howerver you call it), heatbed and a glass sheet of 300x300x3 mm would be too heavy for a single Z axis so I'm thinking of adding a second one and mount them to the side walls.

Would it be better in this case to use threaded rods instead of gt2 belts? (as seen in Smartcore V1.0.1 JSCAD) ?

I'm using M8 threaded rod (single) for my reprap Mk3 heated bed works fine but I can't fully rule out a z axis ribbing from being caused by this design (but more likely my implementation of it). I think you might need to look for a custom heated bed. Thomas Sanladerer does a good video on YouTube about heated beds. You might want to think about your power supply and controls (24V, which board, relays?)