John, the biggest enemy of film finishes is the sun. Spar varnish is made to last longer in the sun, but even so, it will eventually fail. The product you propose is an interior varnish and I don't think you'll be happy with it two years down the road. I know you said it will be under cover, but I'm guessing it will get some sun at some time of the day and in Florida, that will be enough. Someday, you'll be sanding off that flaky finish to try something else. There is something about linseed oil that seems to invite mildue. If it were mine, and it looked as good as yours does, I'd spring for a small container of pure tung oil. Thin only enough to use at once time with 2 parts mineral spirits. It will dry quickly, and most important, it won't require sanding the old finish off to refresh it. Tung oil doesn't seem to mildue as bad as some other oils.
“We should be careful to get out of an experience only the wisdom that is in it and stop there lest we be like the cat that sits down on a hot stove lid. She will never sit down on a hot stove lid again and that is well but also she will never sit down on a cold one anymore.” - Mark Twain