I'm sure I'm missing something here but why not the cardboard pipe they use to form footers?
Easy enough to remove after the pour.

At 12’, that be a big cardboard pipe

And that's what I missed.

Composite decking boards (like Trex) and stakes. Stuff bends like a noodle but plenty strong

Grain bin silos are always poured on a round foundation.

Use the bottom ring as the form.

There is no reason you couldn’t do something similar. Maybe use a cheap 12’ kiddy pool for your form.

^ kiddie pool? What if it pops? Nothing more irritating than a form blow out when you're pouring

It won’t. Hydrostatic pressure is density x depth x gravity… the last one is constant. The density of cement is almost double the density of water (every oilfield cementer knows this). I’m guessing your depth will be something less than 15”…