I Can Replace this Rusted Post but ...

I can replace this rusted post but before I do I have questions!

This is the context:

This construction is about 30 years old. I live in a coastal city so there's plenty of salt in the rain. The ground level was level with the place it's rusted through completely. However, it was probably painted down to that level and no further. My assumption is that it's galvanised steel, but only because that's what's commonly used for this type of construction.

Firstly I just want to confirm that this is likely to be caused simply by being in the ground for 30 years. It's possible that in the past there was water ingress through the screw holes in the roof panels. I had someone replace those fixtures 4 years ago, so those seals are new.

My second question is, am I supposed to use some sort of footing to avoid the same thing happening again in another 20 years ? Should it be in some kind of stirrup like this or are these only for timber:

My third question is, the current top plate / bracket is custom made with welded parts. I don't have a welder nor any experience with that. Should I expect to find a purpose built standard top plate from wherever I buy the post? I could use 3x right angle brackets but that just doesn't feel like the right approach. Presently it looks like this:

Thanks!