That is a really smart insight. I felt like ATS had tougher time blending genres, switching tones than BTVS did. Angel found its footing when it went with alternating the tone for the season arcs.Lonely Hearts is a middle of the road episode so yeah of course I would like something better than that. I'm not saying I wouldn't change certain things in Corrupt, but it probably would have been a better episode.
As for should Angel the show have been darker? No. I think they went dark enough. The problem is being darker has nothing to do with quality. If something is badly written it doesn't matter how dark it is. The same in reverse.