The rear axle spacer swap is only little more involved than removing the rear wheel.
Just for giggles, grab the rear pulley at 3 and 9 o’clock and see if it rocks back and forth.

To answer your question, changing the front sprocket is probably the easiest way to reduce rpms at higher speeds.  Go for it!

Personally, cruising at anything over 65mph on this bike feels unnerving.  It feels like it’s gunna blow up, get death wobbles, or the vibration is gunna make the bike fall apart.  Add a little wind and funky road contour, and who knows where you’ll end up.