Have you used the included gthumb and gimp for viewing and editing? I like geeqie also as viewer, it is available in the repository.
the guys are likely busy with the 7.2 beta2 or rc but you could make a build request for darktable in the new package request threads nearer the bottom of the forum.
both therapee and darktable are gaining popularity as they improve, I think darktable has been in the repository in the past but is currently not available as the new release usually see refreshed repository and as such the request for favorite apps, at least that can be built if someone has time. you would likely be ask to test any builds and get it sorted.
great crew here but one must appreciate its limited size. I note that libraw and libopenraw are also available in the repository if they are required for viewing raw images in gthumb or geeqie, or your installs. the gslapt package manager is on the system menu. hope that helps