All those apps are tied to Plasma, pull Plasma as a dependency, and as such, are not what you'd want to use unless you want to keep 2 DEs around. Okular is an exception, but I know for a fact that Dolphin pulls all of Plasma with it. For an Ark alternative, just use xarchiver (contrary to the name, works perfectly on Wayland). Gwenview is just an image viewer. Use ristretto or something else. COSMIC comes with its own screenshot utility as Spectacle replacement (and COSMIC Screenshot is quite good in my experience). And just replace Dolphin with COSMIC files (for a simpler experience) or Thunar (for something more powerful) and you're good to go.

Edit: But I agree QT theming should be added. IIRC, they've talked to the Plasma team, but it was just too time consuming so they've put it of for now, but will have this done eventually. It's a promised feature, it just probably won't happen soon.

It's possible theme QT and KDE applications on Cosmic. It's just not supported directly by Cosmic. See this post here.