I'm no specialist, but I'll give my two cents. Anyone feel free to correct me if I'm wrong or a better approach is available

1. Yes, even galvanized steel will eventually rust when directly in contact with the ground and it was accelerated by the salty air from the coastal region. It also rusts faster in high humidity air/soil. Even after sealing the roof, it will rust again from capillary humidity from the soil if laid directly against it

2. Concrete or rock footing around 10-20cm above ground level is what I usually see, make sure the water drains and it isn't pooling between footing and post. Those stirrups are usually galvanized steel, but if your post is already galvanized steel, I guess it wouldn't make sense in your case

3. Based on the first image, if you were to use a concrete footing above ground, you might even be able to salvage this post, as the two screw holes look unrusted on the picture, and they seem to be around the level you'd need to cut. You might need to grind down the area and look on the inside to confirm it. If it is, you won't even need new brackets

Otherwise, yes, a local shop will probably have some 90° brackets and bolts too. Keep in mind to accommodate the requirements for hurricane/flood/frost if your region is prone to it

Doesn't look like the pole was gal at all, just painted mild steel. That stirrup will work but will rust again but that's okay as you can replace it easily enough if you are still around.

when you afix the stirrup (get a better one with a foot and chip concrete flat with a cold chisel, use SS bolts, brush and rust proof paint the bit where you cut the support off, just prop the joist with a log (or whatever) and a jack as you are working. It will look ghetto but be solid. You don't want the fasteners taking the load, that's what the foot of the stirrup is for.

I am not a builder, just what i'd do ;)

Steel seems overkill here, I'd honestly just replace that with an NTR-A grade pressure-treated wooden post. Make sure to put the factory-cut end into the ground or treat it with some tar-like wood preservative. Don't use anything water-based as it won't absorb into the already wet lumber.

On the top I'd just use angle brackets since those aren't bearing any load - they're just keeping it in place. On the bottom I'd bury a few concrete patio slabs into the ground and maybe use some kind of steel post feet to anchor it down.