Hi Kis - Looking at the Core XY system I think its overly complex. The rope/belt length is twice or 4x as long as it needs to be (and length means more stretch) for no kinematic benefit that I can see, except that the gantry drive does not need to have a motor on the gantry. I do like belts and have used them in several projects including launching a piano at a theme park and 3 routers. Was a very fast piano.. 60mm wide belt. I do not like U drives as they reverse bend the belt and to be correct, they require very large idlers, so the belts do not fatigue. Since you want a long life, fatigue is important. I think your answer lies in using timing belts not rope and not Core XY. I estimate your gantry will be 6-10kg and the if you place the gantry motor on it, it will be an extra 4kg? So if your clever I think 10kg all up will be a good figure. Since the gantry motor only drives the saddle & payload its got lots of torque available as the saddle and payload will be light (except it will have gravity to deal with in one direction). Since the gantry is driven by two motors again it has twice the torque to do its job. The XY system seems to load share oddly and that may be a pitfall.
I've drawn what I suggest. The benefit of using belts is that they are fully specified so you know their stiffness and their design guidelines. Going rope will be an adventure with various issues to solve. The sawdust image is Brevis one of my small machines. It uses 10mm AT belt. It moves very fast If I program it that way. Around 300mm/sec (18m/min) I also use it for laser work and the speed is useful there. You can get endless belts called ATL which is for precision applications. The teeth are oversize and squeeze into the pulley better... Simple is best someone once said... Peter
edit - Brevis uses one std car on its Y axis each side. This saves weight. You will need medium preload if you go this way. I'd suggest 20mm long cars medium preload one each side.... Brevis is a good light duty router so even a std car with med preload works fine for P&P... Brevis has one bearing for the Z axis and this lets it down. But it was not intended as a router it was for demonstration purposes as a plotter. But every one wanted a router so I moded it to a small router...