Huge carriage sag (>>0.2mm)

[Sebastian Finke]

Does anybody have any advice on how to either reduce the droop or compensate for it?

Removing a stepper (for the left extruder) is pretty much all you can do. That'll reduce the overall mass of the carriage by about 40%.

Are the 8mm x/y axis rails solid or hollow?

I assume they are hollow for reason of cost and weight. That's not a bad thing as hollow rods are more rigid too.

Is there a 10mm upgrade possible?

Not to my knowledge. Its a massive upgrade though. New bearings, carriage, X end rod mounts, Y end rod mounts, pulleys. Would theoretically reduce you X and Y travel by 2mm too. Doable but a ton of work. Is it worth it? Probably not as you would be adding to the moving mass and get even worse prints.

If you're using glass with clips they can warp the glass ever so slightly adding to the discrepancy.