Hi Luke - I imagine you will put the spilt over the central transverse beam? If so not much. The main thing here is the plunging loads pushing the bed down. The beam is doing the resisting in this case.
The XY loads are in-plane to the base so the split does not impact XY deflections much. Making parts for handling is always an issue. I try to limit weight to <50kg for any part.... If the supplier can provide very flat plates maybe 2x10mm thick plates is the go? or use aluminium? Peter
Luke why not start with plywood as the base? A first piece that is levelled as best as possible then a second piece that is levelled with the machine? This becomes your waste board? You describe your machine as a router a 20mm thick steel base is incredible heavy and won't be flat or level? What do you want to do with this machine? I think the 20mm steel base is over-overkill?
Or start with 12mm steel plate plus thick waste board and see how it goes?