I like the FITS Viewer just fine, but often I just want to find a file in the file manager, double-click it, and have a quick look. If I have FITS Viewer associated, it fires up KStars -- a whole new instance, even if I already have one running, which takes several seconds. Likewise it's a lot more clicking to navigate the File | Open | blahblahblah from KStars. I know it's not a huge thing but little user-interface bumps add up, you know?
I think both of those are for Ubuntu. I downloaded the ASTAP source -- I do it for a living, compiling doesn't scare me -- but there doesn't appear to be even a README on how to build it. Probably one buried somewhere on the website, I suppose. Obviously Jasem knows how to do it or ASTAP wouldn't have been an option for solving in previous SM versions!
Siril will work. A bit overkill, perhaps. I use Siril for stacking on a Pi4 sometimes, and I clicked on a FITS file once and found it opened in Siril. The Siril viewer has modes such as Linear and Auto-Stretch, which is also handy. Not good for killing bad files, though.
SkyWatcher 10" SynScan Goto Dobsonian, SV106 Guide, ZWO ASI178MC, Svbony SV105, SV305, Pi3HQcam, Pi4, DIY WaveShare focuser, Linux Ubuntu, KStars/Ekos, Carte du Ciel/CCDCiel, AstroDMx_Capture, Siril, StarTools, GIMP