Stockholm to Trondheim

Very hard to answer, but I can at least give you my thoughts. And I'd say yes, it can be worth it. It will give you a view of the landscape, and especially the part from Östersund to Trondheim is very scenic in my opinion. Further south there are also some very nice views, but also a lot of forests. Another option is to take the overnight train from Stockholm to Östersund and then continue with a regional train to Trondheim (one change in Storlien needed).

If you want to spend a night somewhere along the route, Storlien and Sundsvall are two very different places. Sundsvall is a proper town with a very nice town centre (was voted most beautiful town in Sweden a few years ago). Storlien is a small village in the mountains, mostly known as a ski resort.

Regarding mosquitos, as I mentioned last time you asked about them, it's hard to say without knowing more about your plans.

Is Sundavall worth a full day and say two nights?

If it's as pretty as you say, then it probably deserves extra time.

Worth it or not is in general a very subjective question and depends on many things. But in general I'd say that Sundsvall is an underrated town that is worth a day or two.

An older thread of mine about the poll: https://community.ricksteves.com/travel-forum/sweden/and-the-most-beutiful-town-in-sweden-is

Thanks to Badger's post about the most beautiful towns in Sweden, I added Sundsvall to my 2022 itinerary. Now, I am especially fond of Art Nouveau architecture, so YMMV, but I liked Sundsvall a lot. I spent just one night there (I'm always constrained by the 90-day Schengen limit), arriving from Uppsala and then heading to Stockholm. My total train travel was about 7 hours, so I didn't have much more than 24 hours in Sundsvall. I remember needing to walk fast up and down the streets to see all the beautiful buildings. It would have been nice to have an extra night there, but whether you need two nights will probably depend on your previous stop and the following stop (i.e., travel time) and whether you feel the need to try to see every single interesting building in town.

Alesund, Norway, is another fabulous Art Nouveau town.

And of course Stockholm is gorgeous, with plenty of Art Nouveau (and other) architectural eye candy.

Thanks. I'm more of a skimmer rather than someone who has to stop at each building or exhibit.

This will be my third visit to Stockholm.