You can make yourself less vulnerable but not invulnerable.

Here in Florida we have our fair share of tornados. A lot of people who build new homes, or are re-doing their current home put in shelters. They're actually above ground. They'll pick an interior room such as a closet or a pantry and make that room a concrete box which is bolted to pylons made of concrete they drive roughly 6-10' into the ground. The ceiling and the walls of the room are around 16" thick rebar saturated concrete with a heavy steel door.

Basically your house blows away, but you're in your shelter fine. Florida's tornados tend to be smaller. Small ones that do about a few hundred yards of damage and then disappear and/or reappear miles away. So I would wonder if a Florida Tornado is less strong than a classic Midwest Tornado. Probably because of all the ones I have seen or heard were EF-2 at the highest.

I've been witness to some tornados in my life. Scariest and most rapidly occurring thing I have ever been part of in my life.

That Cozy Cabin looks good.

Be aware that the main exit may become blocked by debris, so consider how you may get out that air vent in an emergency (tools and a ladder).

Also consider taking tools with you so you could bash your way out the main entry (car jack, pry bars, axe, etc)

I imagine you need to site it so that flooding is not a risk either.

I am looking into a bomb/storm shelter underground. We have lots of topography condusive to this, but my issue is TOO much bedrock.