I've heard fluid film is about the best of the self-service undercoats and if rust has started anywhere it will slow its progression significantly.  I use it on the underside of my son's Jeep Grand Cherokee which has spent some time up north and has inherited some rot.  On the downside it is very greasy and messy, and does not "dry", so you will probably want to avoid the exhaust and the catalytic converter.  Also, the smell takes a little getting used to.  I think it will last about a year, so your plan to redo it every fall is a good one.

If you make sure your vehicle is rust free and cover it with a thick coat then it should be plenty good and you shouldn't have to deal with rust for quite a while. The reason you want to make sure its rust free before coating it is rust does spread and it can spread pretty fast and if you just spray over the rust then it will continue to deteriorate under the coating making the spray practically useless.

I hear great things about fluid film. It supposed to be better than traditional rubber undercoatings which can trap the any existing rust underneath and make matters worse.

Ya, I got a Milwaukee M12 angled die grinder thingie to try to get most of that brown crap off before. And a metal brush.