Your tags about how it’s not about the Collector being powerful and how their biggest enemy is themselves really make me think about how this all likely plays into the shortening. Had the show not been shortened, we would’ve gotten a Collector with a different personality (as Dana confirmed) who wouldn’t have had that weakness and so they would’ve been a lot more difficult, taken more time and setup to defeat (assuming they didn’t just stay trapped, which could’ve occurred).
But with the shortening, the Collector is so much more negotiable and so the writers don’t need to cook up a well-paced raising of the power scale, they just need to have a heart to heart and they’ve already done that with Amity and Lilith and whatnot. Giving the Collector a backstory with King and their interactions over the timeskip helps.
There’s other confirmed reasons for the change in personality (wanting King to have an immortal friend he can relate to which probably wouldn’t work out if the Collector was the one who killed his family, as well as Dana having a character concept she wanted to try out that could fit with enough tweaking) but I think this is a legit consideration.
Yeah, honestly I'm not sure what Dana's ultimate plan with Collector was especially with the context of wanting more Coven Head stuff, but I just really enjoy the power scaling in the series and I don't think Collector ever was going to be the main antagonist! Belos fits so well with Luz's arc and what the goals of the show were. I kinda think they would've spent more time with Collector but I don't think we could ever gotten to the power level to fight them. It always had to be a conversation.
King could defeat Collector technically but I think Collector is just so ridiculously powerful that even adult King would struggle. and I'm really glad they didn't do the trope of having Collector be secretly more evil than Belos. I'm really glad he's the final boss, it works so well with Luz's arc and her taking back her power and self-love. :D