No expert either. These trees did they come from USA? Did they have earth around the roots?
I ask this because coming from the wild will have the roots infected with mycorrhizal fungi, which is a good thing for the uptake of nutrients.
The fungi attach to the roots and help the plant get nutrients the roots can't get in exchange for extrudates, sugars the plant photosynthesizes.
If the plant had only new european
compost around the rootsystem it can get all the nutrients it needs for growth easily at first, the plant doesn't form these exchange programs with fungi, because it doesn't feel it needs to, but after a while the ground is nearly depleted it finds it should have. Since yours are inside most of the time chances of spores finding these roots are slim, I've seen big Sequoia's in gardens in France, so beneficial fungi must be around maybe Stockholm has a botanical garden with one in it?
I've read that Abies Alba can have as much at a hundred different kinds of mycorrhyzal funghi, that could be another chance.
You could look for such a tree take some soil from under there, doesn't have to be much, one garden spade full contains lots and lots of beneficial fungi. Mix it in water and give that water to the plant.Or ideally another Sequoia.
I'm far from an expert, but for trees fungi are more important than bacteria.