what's up with the big tree?
Outside United States forestry
I am a Certified Municipal Arborist through the International Society of Arboriculture. I work in North Carolina, USA. From the photos sent, I cannot discern the symptoms or condition of this tree.
My recommendation is to contact your local Finland chapter of the International Society of Arboriculture for a list of local Certified Arborist who could make a site visit to determine the best path of care for your tree.
The main page is: https://www.suomenpuunhoidonyhdistys.fi/
The page for arborist information is: https://www.suomenpuunhoidonyhdistys.fi/arboristi/
There is a list of Certified Arborist at: https://www.suomenpuunhoidonyhdistys.fi/arboristirekisteri/
This sounds like an advertisement for ISA, but their membership is trained in tree care and usually are conscientious of principles of care in the tree care industry.
I hope this helps.