annual top-up after initial treatment is the way to go. At some point a replacement subframe will make more sense. They aren’t silly prices.
I will not add to the good advice given, but from my experience, do the box sections with Dinitrol ML, the sills, rear subframe, etc. It’s a very messy job as the stuff flows out so place lots of newspapers underneath. Do this every year, inspect the underneath for rust breakthrough, and retreat as necessary. However, not using the car on wet days, may not be possible, does help!
Sorry for the late reply but one other piece of advice: Concentrate on the monocoque chassis (particularly any mounting points for subframes/running gear) and the big subframes for the most economic preventative maintenance. Control arms etc can be bought cheap second hand and refurbed and replaced, but if your chassis mounting points corrode then some expensive welding will be required. Basically, prioritise everything that is sprung weight before anything else, even unbolting to treat between the joins is my approach. I will not dissuade you from doing the running gear while you’re there, though, as it looks great and can only help! Keep it up!
Get up into that wheel arch. Undo the top mount of the shock and whatever control arm you need to make it sag down, then abrade and treat. A tough and dirty job, but by far one of the most worthwhile.