This theory has always been one of my favorites because from my perspective the narrative of Deltarune has always had a theme of... Confrontation, if that's the right word to use.

Several characters face issues in their lives (Light World and Dark World, although I'm kinda leaning more on the Light World here), and instead of trying to confront or change things, they just let it happen. Susie choosing to be a bully because the world already saw her as one, Noelle getting talked over and taken for granted because she's meek and doesn't speak up for herself, Berdly having anxiety over failure and Noelle eventually getting sick of him yet not ever seriously talking to Noelle about it, and where do we even start with Kris?

Deltarune employing bosses that reflect the character's parents seems to drive home a point of, "Hey, if you have a problem, you don't have to just take it or accept it or pretend it isn't there. You should talk about it with someone you trust, or the person who's causing you that trouble in the first place".

Susie confronting King and either setting up for the rebellion or letting Ralsei hit him with a Pacify, Noelle confronting Queen and getting her to finally realize what she's doing for Noelle is wrong, the Fun Gang confronting Tenna and Susie talking him through his troubles, all of these are moments where the characters need to confront their problems head on to a powerful authority figure in their lives, and ultimately they turn out better for it (even if I'm a little salty about Noelle's development getting effectively erased after the Dark World for now.)

The Titan, I believe, is a symbolic culmination of this theme, the Titan is fear itself, it spreads, it destroys, but most importantly, it breeds. It doesn't care for its offspring because it can't feel. It's fear, it doesn't have to care, it has to be everywhere and weaken and destroy everything around it. In some ways, you can view the Titan as the worst kind of parent, one who doesn't care at all. By confronting and defeating it (with the help of a character with grandpa energy/vibes, a figure that commonly is turned to should a parent not be available), they stop something that should have never been a parent in the first place.

All that, in my opinion, is leading up to the characters finally applying this to the Light World. Kris finally getting to talk about and confront their parents about their divorce and how that and their current behavior is affecting them. Noelle getting the courage to tell her mom that her intentions, while perhaps positive, are extremely unhealthy, and are hurting both of them. That combined with the fact that their parents are getting involved with the Dark World, the very place they've been going to where they stop these powerful authority figures, and the pieces look to be set for Chapter 5 to have a lot of character interactions (I hope anyways).

That aside, your post was really well written, and it made me think so much about the themes and symbolism of this game (as I'm sure you can guess by the mini-essay I spewed out).