There are some really odd design choices here. Barrel shape with large doors. That does not appear to be well thought out design. I’m really looking for either plans or prefabs that are well thought out and designed.
These are pretty much standard
https://finnishsaunabuilders.com/collections/outdoor-saunas
and just Google "Finnish Sauna" and you'll see it's mostly shed forms and barrels. A few Kota shapes are thrown in.
http://localmile.org/blog/2020/11/05/notes-on-building-a-sau...
Basically there is not enough room in the bottom for cold air and the cold air Gradient is higher than it should be.
There are all sorts of other design considerations, that impact the convection and air flow that i dont think these ever even thought about