Sounds good then! I'd try running it hot after use -trick to see if it makes any difference. You could probably get good 50 years of use out of it if you treat the wooden parts inside with oil such as [1]. The sauna should be extremely dry before treating it and you can do that by running it couple of hours hot without using. The oil needs to be non toxic and non water soluble so high temperatures won't vaporize any toxins from it to your breathing air. It deepens the color of the wood a bit but should look even better.

[1] https://www.amazon.com/Liberon-003815-Finishing-Oil-Liter/dp...