>Doc Martens platform boots alternatives?

Yes.

>I'm specifically looking for half calf platform boots with a zipper on the side, and they need to be available in size 12.

Tricker's! They sell similar boots to Dr. Martens, and they offer bespoke handmade shoes. They supply all sorts, including the U.K's royal family.

>I have $200

It would be a stretch to find any boots at that price that would be truly BIFL.

You can get mid-calf platform boots with a zipper from New Rock, but probably not BIFL, they're ~$200. Some say they last long, but that's people who I suspect bought them 90s/00s. I have read there have been recent declines in quality.

To be clear, I don't own shoes from any of these brands, this is just information gathered from friends and research. Grenson is another alternative that doesn't meet your list of requirements, but probably outlast New Rocks and DMs.