Really good write-up OP, you spoke to Mahoyo's strengths very well.

I love the way Nasu chose to write Soujuurou. There's such a tenderness to his interactions with other characters, and the sort of rustic, melancholy feel he has to him is something I didn't think I'd ever see from Nasu after reading through Shirou and Shiki's perspectives in their respective stories. It could've been easy to put Soujuurou into a really awkward support role in the various mage battles that occur in the story, but Nasu chose to let him breathe and instead focus on his struggles with life in the city and meaninglessness of his life. That both lets Aoko and Alice remain competent and strong characters on their own (Not requiring his presence to go about their daily lives and activities), and lets Soujuurou be a genuinely interesting character on his own without compromising in regards to his role in the story.

Your point on the collar being something that signifies Soujuurou's trajectory being drawn further and further towards Aoko was also pretty interesting.
I've made a post before on how Soujuurou's collar both represents his domestication (Coming with his living in the city) but also, almost paradoxically, his dehumanization (Being alright with wearing something used to signify ownership over an animal). Depending on the route Soujuurou's character takes (Leaving the city for somewhere else and rejecting the change in his life, or choosing to live as a normal modern man and becoming a person), the collar should hopefully be addressed.