It's good to see Sakura again, and I'm sure many characters will be happy to see her IC, as well! I think you've got a solid plan for managing two characters. For upgrading her psychic abilities, there are two options. Because that NPC's psychic power was Mental Connection, upgrades and pins that apply to Mental Connection in Psychonauts 2 could apply to it.
[url]https://psychonauts.fandom.com/wiki/Mental_Connection[/url]
Second, she could lean into standard SAS abilities like she already has with Brain Drive. One example is Brain Crush, allowing her to force open a port to a mentally staggered opponent with her SAS cables and deal a huge burst of psychic damage as a finishing move. Another is leaning on her psionic friends from Midgar, gaining their abilities through SAS, either to use their powers for herself or to conjure Visions of them for offense and defense. So there's plenty of options, and I'd be happy to write your choice up for you if that would be convenient.