Maintaining constant velocity while switching on an output

I increased the acceleration significantly and it seems to have helped but there is still a very clear slow down stutter between moves.

Are you sure you tried it with M62 and M63?
It doesn't seem to slow in my simulation, (It does slow with M64/M65 and M8/M9)

I had been using both M62/M63 and M64/M65 for testing it out. It looks like another reset of the system seems to have helped fix that last little bit of slowdown. Don't know how and it doesn't make me confident that it will stay that way but I'll take it for now. Thank you all for your help.

There is one little thing you must remember about M62/M63. They require that a move be commanded after they are issued for their changes to take effect.

Assuming you are moving along a single linear axis for painting, another approach would be to not use gcode to enable your spray.

Instead fire it using a hal component that monitored axis.L.pos−cmd and had an output pin that enabled the spray based on specific axis coordinates. Eg. turn on at 3" and off at 4". You could still do it for multi axis moves, but there would be a bit more maths involved. This would totally eliminate any possible issue with gcode affecting motion.