Hi Milo,
I am also going to Stockholm this summer, so I've been collecting recommendations from various threads. I'll mention a few and provide the threads where I've found those, with the caveat that I've not tried any of them.
"Citysallad on Skeppsbron is still open according to their website. If you're willing to spend a bit more on lunch, Slingerbulten on Stora Nygatan is a good place for lunch. They serve traditional Swedish food at a reasonable price. The lunch menu varies, but they usually have meatballs every day."
From https://community.ricksteves.com/travel-forum/sweden/2-days-in-stockholm-f89faf1b-50d5-42cb-89f9-c6173025ab40
"When I meet Swedish relatives, we go to Vettekatten, a Stockholm bakery / fika / lunch institution, about a 7-min walk from the central train station. Try to get there before or after the lunchtime rush."
https://vetekatten.se/en/
https://community.ricksteves.com/travel-forum/sweden/visiting-stockholm
I have a reservation at Prinsen, which is described as "classic Swedish cuisine in a venerable restaurant with a wood-paneled interior & ornate chandeliers." I can't recall where I found that one.
And I've heard good things about Grand Hotel Veranda, offering "a smörgåsbord with all the trimmings."
I just found this older thread, which also recommends Grand Veranda. Hard to say whether some of the other restaurants mentioned may have closed in the 5 years since the post.
https://community.ricksteves.com/travel-forum/sweden/looking-for-a-good-swedish-restaurant-in-stockholm
Hopefully, others who know Stockholm will give you some additional (perhaps newer) ideas.