When I bought my toledo, it looked fairly tidy
Some time later, on a friday (I think) I saw a small bubble on a sill. MoT was on the Monday.
I poked said bubble, to discover a thin skim of filler over silverfoil type tape. It had been like that for several years......
(I spent the weekend stripping the tape/filler off the arches, and cutting out a lot of metal/rust, then making/welding in repair panels)
Anyway, my point is that ignoring the naughtyness of such repairs, it held up over MoTs etc and looked fine. So for your needs, something similar will work.
Personally, I would scrape off all the scabby rust/crud. Possibly some tape to give you something to fill to, then use Upol Fibril (wonderful stuff, a fibreglass based filler but way "softer" to use than P40etc. It is in the rain gutters of my toledo, and has been for 8, yes 8 years with zero bubbling/further issue.
I like cars that are patched up and used. To me they are having a much better life than those that are left forever locked away.....It is what they were born for.
Passing an MOT is not a measure of the quality of the repair. It should fail, it was hidden well enough they couldn't see it.
Once something has started to go, you will struggle to stop it unless it is something superficial like surface rust. Once it has penetrated to the surface, you will usually find it had spread 10 times further on the inside of the panel.
I have tried every products and bodge in the past. They have all failed, even on panels when I thought I had got away with it. When the bodge was removed there was a horror show underneath.
Unless it is surface corrosion, the only way to fix it is to cut it out and replace it.