Sauna and wellness visits

The question you need to ask yourself is: to what extent will you use the private sauna for professional purposes? How will the sauna demonstrably improve your self-employed activity?

I don't want to answer the question for you, of course, but I fear that it will be difficult to attach a professional use to it :) And consequently, the sauna will not be allowed in your accounts.

I would say that because you do heavy work and your back needs pampering, you should go for a massage... then you can work longer and your back will last longer.