Myles,
There are now reports that that even though there is still a lot of snow at Tjäktja, its only for a few kilometres and there is a path in the snow so they have very few people turning back. And its still a few days before you get there.
Having said that…
I have tried to answer your questions rather than state my own preferences, but now I have to say that I would prefer your backup plan with Vistasvaggi myself, specially now when you have started to think about interesting daytrips. To go to Nallu (Stuor Reaiddavaggi) is an excellent way to go into one of the more narrow valleys without difficult water crossings. There is a crossing just before Nallu but that should normally be an easy one, suitable if you are inexperienced with crossings. Remember that you cross where it is wide and that if snow is melting it may be a bit more water on the way home. You only cross if it feels comfortable, and if not, you have already reached a long way into the valley and close to Nallu, not to far from where you would have to turn anyway. Check with the manager of the Vistas-hut about snow in the valley but it should be ok I imagine, the valley floor is not that high up. A recommended day trip.
An alternative from the Vistas-hut is to follow the trail to the Mårma-pass up on the other side of the Vistas valley. It is south facing so it should be relatively snow-free. you will a view into the higher mountains and valleys on the other side of Vistasvaggi more than worth the effort. (But don’t decide to continue over the Mårma pass…) That is a shorter trip.
But there will be good day trips along the whole way, so you can stay flexible and decide based on weather and mood. (Though not many as good as Stuor Reaiddavaggi.) My best advice is to ask the managers of the hut about day trips. Most of them will be very helpful and they have a lot of knowledge of the current conditions to base their advice on.
Stockholm is well worth a couple of days. Don’t miss Gamla Stand (Old Town) and if you have time and want to see something different from the city centre, go to Fjäderholmarna where the archipelago starts, its only 30 minutes by boat. (But I understand you probably don’t have the time for that.)
https://www.stromma.se/en/stockholm/excursions/day-trips/fjaderholmarna/
What ever you go for, I hope you will enjoy your days in Sweden and in the mountains, and please give some feedback how you found it, a trip-report maybe?