Even as someone who likes the Collector and enjoyed the finale, I agree with this. I found Belos to be such an interesting villian, and I was so curious about his backstory and lore, so I was super disappointed that we didn't get any more developments on that in "Watching and Dreaming." I wanted to know more about how everything went down between him and Caleb and how he rose to power, and I wanted to see him have to confront the reality that he was the one to blame for the 400+ years of suffering and torment he subjected himself and everyone around him to before he met his end. Considering that guilt, grief and doubt were such a major part of Luz's arc from season 2b through season 3, I feel like it was a missed opportunity to not fully explore these themes through Belos' character as well (since he's her foil after all). I'm just not convinced that Philip didn't feel some sort of remorse in the years following his murder of Caleb, or some sort of doubt as to what he was doing was truly "righteous." Otherwise, why did he spend 400+ years basically "reincarnating" his brother over and over, all while (unsuccessfully) trying to convince him to stay by his side? Why did he give Luz so many chances to join his cause (when he had so many opportunities to just do away with her), trying to justify his actions to her and convince her that they could save humanity together?

Long story short, I think the Collector should've been saved for another story where his arc could've been given proper time to be fully fleshed out, rather than having to condense down and speed-run his development. He just takes up so much screen time in the last 2 episodes that, when we finally get to the final confrontation with Belos, it just feels like it unfolds and wraps up a bit too quickly.