As I am using an Australian / Asia-pac router, I’ll make the comments general. The base plate (router holder) and fixings need to be durable and avoid distortion. Further I’d consider the management and maintenance (although ideally periodic and infrequent), the ability to easily access or dis-assemble components without a full breakdown! If you’ve ever worked on a car or other industry tool and required the SST (Specialised Service Tool, or welder, old tool / wrench and a plasma cutter ;-))
The underlay ideally would be ‘changeable’ although not initially over time that material will wear. To one of the previous comments, although reducing material, I think an edge maybe more subject to damage and harder to replace repair than a plate, the plate does not need to cover the entire bottom, I took the old friction plate of my router and attached it to the sled bottom, adjusted slightly for CoM. If the edge can be easily secured and replaced go for it!
What you don’t want is it wearing or ‘marking’ the material you’rer cutting, that will result in more ‘prep’ work for the finishing. Temperature stability would be key, e.g. try it with a garage door open in Alaska and Mexico winter and summer respectively!
To the wood v. metal, the consideration of Aluminium may also provide some options, I take it you’ve avoided a casting or poly-carb material from consideration, looking to cut the parts if required from the CNC?
The total mass, unless your objective is to remove the ‘brick’ will add load to your motors and drive, as well as momentum as it’s moving, remember our friend Newton.