Let's say I have a Tera Stellar Vileplume with an Expert Belt that uses Tera Blast on another Terastalized Pokémon. Would Tera Blast get the Expert Belt boost, since it is super effective on any terastalized Pokémon?
Yes. The Stellar type has no unique characteristic that prevents the Expert Belt from working.
Expert Belt (Bulbapedia) Stellar type (Bulbapedia)