Underseal ,Rust prevention. Is it worth it. ?

As the owner of a Jimny JB74 that is less than a year old, I've read the various threads regarding rustproofing with interest.
I then look at other threads with examples of severe corrosion on the bodies of older Jimnys. It strikes me that a lot of the corrosion shown seems to develop on the inside of panels and works its way outwards. This suggests that rustproofing applied to the outside of the vehicle may not be as effective as hoped.
Maybe trying to avoid condensation within the vehicle and dealing with any water leaks are just as important. I'm also very diligent in removing mud that gets trapped in certain places under the vehicle. These "mud poultices" are a great way of encouraging rust. As with virtually every car I've ever owned, a common place for mud to build up is the lip on the edge of the rear wheel arch.
I often see proud car owners washing the well-protected, painted bodywork of their cars whilst ignoring the muck building up on the underside.

From my observation of the Jimny, the rust starts outside and creeps to the inside. So, as written before, a good rust protection as early as possible extends the life of the bodywork considerable.

New cars are a lot better at resisting rust, it's rare nowadays for a car to be scrapped because of corrosion.
l once had a new Alfa Romeo Alfasud that was scrap by the first MOT "nothing left to weld to" said the MOT man.

Having paid something like £8,000 new three years previously l got £250 for it.

l've treated what looks like vulnerable parts of our new Jimny with rust prevention products, but to be honest at approaching 60 if l am still alive in ten years time when the Jimny might start to show evidence of corrosion, then l will be more than happy.

Having my 1 year old treated by main agent Suzuki with Waxoil at what I think is good value at £120 plus VAT. Normally I would not go to main agent but it’s going to be done in their own body repair place. Unfortunately my SX5 was sideswiped by a new young driver pulling out into me He had to except liability , have to say the side of Jimny stood up well considering the crash noise on my dashcam. After they have resprayed paintwork they say they will do the messy job when no cars nearby ! They suggest 3 year touch up. Guess this makes sense ?

I've not had a quote for less than £500 +vat. Hence I've been looking into doing it myself. Even buying ramps/jacks and all the kit I'll save a fortune, but I'll have to do it outside which is putting me off

For £120 plus vat at a main dealer, I would be asking them when they could fit me in and could they do my wife's car too.