Kungsleden is a big motorway of a path with hundreds of folk walking it. Several of the huts have electricity, but I'm not sure about mobile phone access. It's substantially cheaper to camp beside a Swedish hut and use its kitchen and lounge than it is to sleep inside.
You won't need 30-40 days for Kungsleden, unless you diverge and spend time around the high (glaciated) mountains around Kebnekaise. In that case you'll need a tent. If not, you could continue after Abisko on the so-called nord-kulotten (northern cap) along the Swedish-Norwegian border, which briefly enters Finland's panhandle and can be extended to Kautokeino on Finnmark. Here the huts are much smaller and more primitive (no food) and can be over-filled by commercial treks in August (no booking needed), so again a tent can be a good idea.
You are right. You'll be too late for the midnight sun, but you may still be a bit early to see northern lights, as twilight still lasts all night at that latitude, especially in August.
An alternative, if you want to walk light, could be to take the bus from Oslo to Haukeliseter on the southern fringe of Hardangarvidda, then walk to Jotunheimen. Huts all the way, so you just need your day sack plus toilet bag and a change of clothes. Lots of scope for variation, especially in Jotunheimen, and you could continue northwards into Breheimen and ever onwards.
Sarek is very wild with no facilities at all (including paths). Some big river crossings are necessary, too, so not a place for solo walking. I would advise having some experience of less demanding parts of Scandinavia before tackling it.
Lofoten is very dramatic, but the scope for walking is limited, as the mountains are so steep. Great place for short walks, though.