2018 rear axle slot in swingarm
 

I feel this slot is machined too short - it stops way short of alowing the tire/wheel to be pushed forward to where the tire contacts the shock flap/guard. Cutting that slot only 1/3 inch longer would allow better fine tuning of the wheelbase and, more importantly, make changes to gearing possible without always having to change chain length. Should I attempt to make the slot longer and if so what type of cutter would be best?

Making the slot bigger may weaken the swingarm.

Use a burring bit to remove the aluminum.

https://drillsandcutters.com/sa3nf-3-8-x-3-4-cylindrical-carbide-burr-1-4-shank-aluminum/