(If anyone cares, this response has some vague spoilers for the ending.)
I wanna preemptively say that I'm not arguing or saying you're wrong here; I'm not fully happy with the ending either. I genuinely just really like talking about my art and the reasons I make certain choices, and you're the first person to bring up the ending. (You can still think the reasons are bad.)
0. Thank you for playing and for your kind words! I really enjoy writing so I'm glad this slight dip into a narrative-focused game is resonating with anyone.
1. The exchange mechanic is meant to keep people from sticking with one combo for the entire game. Certain pieces might be more favorable to you, and I like the risk/reward of deciding whether to use them on an easier boss or wait for the chance to have them at the end of the game. Some bosses are already easier with specific movesets over others, though, so this might not have been necessary.
2. I mostly wanted the ending to be "disappointing" for thematic reasons. For one, gameplay-wise, it's hard to have a slow progression when the game is structured like it is. The format of "here's a specific insecurity, fight it, here's the next one, fight it" just didn't lend itself well to a traditional narrative. It wouldn't make sense to have the protagonist realize it's okay not to have a connection to their own sense of humor, for example, and then have the exact same conundrum over their talents. Having another ending would have required a really harsh flip in tone at the very end of the game.
From a narrative perspective, it's just impossible to say where an arc like this is supposed to end up. I feel like the "happy" ending that most similar stories present is the main character finding out the one thing that makes them unique, but I really didn't want to go that route. For one, it just isn't a realistic finale for a lot of people; it's much more common for them to be forcibly squeezed into a specific role they have no interest in just to survive. As people grow up and are forced into deciding their one area of study and their one career path for the rest of their life, the stakes become too high to really scrutinize their choices this much. Self-satisfaction and happiness become less important than just getting through each day with the money needed to survive. The game doesn't focus on these larger societal issues, but I still wanted to avoid a message that portrays the issue unrealistically.
I also don't think this style of happy ending gets at the heart of the issue, which is that everyone is just a collection of traits that they got from other people, and this is what makes us people in the first place. I mostly incorporated this into the bosses themselves. Even if they appear as individual, cohesive designs, each of them have different art styles for different attacks. Sense of Humor's pie attack shares an art style with the final boss, and its cross attack shares an art style with Talent, whose music attack shares an art style with Sense of Humor, etc. The protagonist is so wrapped up in their own insecurities, though, that it doesn't really cross their mind. (This is another reason I didn't think it would make sense to suddenly spell this realization out at the end of the game. They're just barely starting to realize self-acceptance is even an option as the game ends.)
TL;DR This inner turmoil affects real people a lot, as you pointed out, it's not something that you just get over one day, and I wanted to reflect that with an open ending that implies there's a lot more growth to undergo.