I’ve been thinking about sag (the wife’s getting on…)
The bricks perform two functions: holding the router to the material; assisting gravity in overcoming chain sag.
Too much mass reduces the ability of the sled to react to changes, making things worse. Too little mass and the weight of the chain overcomes the tension from the bricks, making things worse.
So - things to do to make things ‘better’ from a motion-control perspective are to: reduce the weight of the sled - less mass is always better when you’re moving things about; remove chain sag from the equation by not having chains acting against the motion by not having them hanging over the workpiece and only operating in a linear vertical motion.
For this you would either need a non-stretching, lightweight leader attached to the chain/motor drive or a different method of motion than hanging the sled from the two top corners. As the second one would mean it’s no longer a Maslow, that’s a non-starter.
To traverse a full 8x4 sheet of material the distance from the top corner to the furthest is ~2700mm - add in a bit for frame, etc. and let’s say 3M of leader is needed. To match that 3M of leader is 3M of chain, to give you the full range of motion.
Currently the chain is tensioned using shock cord, but that doesn’t have to be the case - gravity and a bucket ‘o’ chain can serve to act as a reservoir - but you need positive chain engagement so the chain won’t slip, rather than relying on tension from shock cord and an acute angle to keep it in place. Two simple sprung rmotorcycle-type chain tensioners either side of the motor should suffice to keep it in contact with the sprocket.
To keep the sled in contact with the material and lose some mass, use a shock cord of sufficient length running on a traveller (like the boom on a yacht). This would ensure only vertical pull is exerted on the sled, while overcoming both the lighter sag potential from a light leader and the spindle/cutting tool.
Other advantages of a leader/remote motor arrangement could be: easier implementation of a limit switch, giving you straightforward calibration; greater range of motor options; less mass means faster motion; the freedom to place motors anywhere means they could be co-located on one side of the machine, making for simpler wiring - they can even use one single chain between them, as long as it allowed for full-extent movement.
Using a ‘traveller’ with shock cord makes for a slightly more cumbersome loading of material, but I would just have it hook on the sled and unhook it for material changes. For third-world/cheaper implementations both the traveller and leader/tensioner/pulley could use cheap ‘shower door’ running gear, as there is no need for high accuracy in those areas.
Moving away from the current worm gear to a NEMA standard, from a packaging perspective, gives a lot more options to the user - you can source NEMA steppers/servos across the world, and they are designed for this sort of installation. Supplying a NEMA-23 standard worm drive gearbox instead of a motor gives greater flexibility and removes the need to make the kit other than generic; NEMA steppers are available from lightweight to industrial tough, with huge duty cycles. The worm drive gearboxes are a significant step up in quality from the door-opener type boxes you are currently using.