I see varnish as a sealer so an 'overlap' of the rail/deck joint, is a good idea. It's invisible except very close up. Other than that, the only trick's I look for is how to make the task most comfortable. You're working in the water which means you have to work on the deck quite a bit. Plus you have stanchions in place. That sounds tough!

I try to do that task from staging around the boat(simple planks, horses and step ladders). Ideally, I'd put myself at a standing height that puts the toe rail about a foot above my waist. That way I can scrape, sand, tape, varnish, from a standing position.

You could achieve similar in a dinghy and some from the dock(maybe a short plank from dock to dinghy?). The marvel in watching boat builders work is how they stage themselves into position to do the work. They create work spaces and levels around the boat that are quite amazing and super efficient.