Didn't think anything of it and started Shiori's path in AE21 despite wanting to see everything from every character as well (didn't set up saves since I didn't think it was needed). Does that lock me out of the rest of the character's events completely, thus requiring me to restart from the beginning in order to see everything, or am I still able to see everything on this save?

Yes, you won't be able to play each of the routes simultaneously on a single save. It's a deliberate choice on our end. Each of the main routes are about Keisuke's romance with a girl of player's choice. We don't have plans for harem stories. While the early parts of the game allows you to play scenes from each route and get a feel and tone of route's characters, after making your choice to commit to relationship, other routes will remain locked. The girls from each of these routes are still a part of your route and optional events (crossover scenes with other girls, special scenes, MC scenes with events focused on main character's slice of life or RM events which are focused more on learning more about the lore with Kei's roommate Daichi), but the main story will always be focused on a romance with a girl you picked