I wary of powered, flat surface sanders on rounded hull shapes. If the sander has a soft pad option, that might be safe-ish.

Since you're proposing a light sand and re-varnish, is it reasonable to assume you're happy with the general external appearance? -this is a protective, longevity-extending coat, not a remedial repair nor a catch-up for some past deferred maintenance? If so...

I'd be inclined to a some hard rubber quarter-sheet sanding blocks and wet sand with 150 grit to start. Your first "cut" into the varnish wants to be aggressive enough to remove the outer dirty or oxidized layer, but not necessarily remove all the varnish. Aim to get sanding scratches on ~95% of the surface. Add a coat (or coats, if your varnish system allows wet-on-green rapid recoating), then switch to maybe 220 grit and wet sand to achieve maybe 98% scratch pattern on the surface, another coat, sanded with ~320, almost 100% of the surface, then clean, tack rag, clean your workspace of all dust or lint, clean yourself, comb your hair in another room, make sure no bugs can get in, and apply your hero coat based on everything you've learned in earlier coats.

Note that many varnishes benefit from a capful or two of thinner when applied to large surfaces. It allows extra time to complete a swath and return to the beginning side and still have a wet edge. For a canoe, you may need to accept a dry edge along the keel/centerline.

Dispense varnish through a paint filter into a working cup- don't dip out of the can. Foam brushes work fine. You can roll and tip, if you have a helper. They can help spot the holidays and sags, if they're rolling, then watching you catch up with the brush.