Good reminder, thanks - I guess two bricks is around 5-7kg so much more variation there than changing for a lighter router.
I have some 20t sprockets on order and so was guessing that double the speed would like to be around half the weight. With the mechanics page giving an ideal weight of 18lbs, I was aiming for 10lbs, which I think is less than the weight of the Ridgid 22002 on it’s own. A kress is about 4lbs, leaving me plenty of room for the sled with a UHMW base, z-axis, ring, vacuum hose etc. My current worry is how much is enough weight to push the bit into the material. Of course the other driver for low rpms is the relative silence.
On the other hand, if the max force on the motors is at top-middle, then raising the top bar higher above the workpiece would probably be an easier way to avoid any motor strength limits - even before the new driver board and more powerful motors etc.