I'll be making a return visit to Stockholm on Wednesday.
I've already been to the Vasa Museum. That was in October of 2018. Walked right in in the early afternoon. Not sure if May will be that busy but July might be.
If you have time, or time it right, the museum has nice cafeteria and the food was good.
No interest in the Abba museum.
However, I would suggest a visit to the Skansen --the open air museum. It is on the same tram line as the Abba Museum and the Vasa. I was there off season so many things were closed but I still spent about three hours there. I visited the Vasa afterwards.