The dimensions of your base plate will mostly be determined by the diameter of your pier and the anchor bolts you choose. You need to have enough room to use a wrench to tighten the nuts down during installation.
Gussets are likely not necessary for a pier only 100cm tall.
If your fabricator has some scrap stainless he would sell you cheap, go for it. Structurally there is no benefit. I wouldn't worry about weight, either way. You are making a permanent installation, so who cares how heavy your pier is? As for making it heavy on purpose, the benefits will be negligible. Rather, aim to make a secure connection to a good footer/ foundation, and making the pier itself stiff and rigid.
As for the pier you posted in #24, I don't like the separate top plates. That's often called a "rat cage" and is used for leveling the top plate independently of making the pier plumb. You can also use it for routing cables through the pier to a mount without through-the-mount cable provisions. However, there is expense in buying and assembling all those parts and it isn't necessary. If your pier is fabricated will, making it plumb will also result in a level top plate. If you want cables routed through your pier (I highly suggest that) and your mount doesn't have through-the-mount cabling, you can have a couple of holes made in your pier to allow for cable exits near the top. It won't affect the stiffness of your pier.
As for rust prevention, stainless steel is of course inherently rust proof. Galvanizing will work fine for a steel pier. It may, though, be easier to find a painter or powder coater. Then you'll have your choice of colors, too. The direct to metal (DTM) primers are getting good, so you could also paint it yourself. A good paint should last several years even outside.