Are you using the formal or informal definition of imperialism here? Because I don’t see how it meets the formal definition based on my understanding of the underlying economic mechanisms. For example, if they were engaging in imperialism why would Tibetan’s own their own homes instead of the land being taken over and the average peasant being proletarianized/forced into urban centers or large scale production agriculture to work?

Why would them taking over Tibet be expansionist? Tibet has been part of China for hundreds of years, only breaking off briefly during instability caused by foreign imperialist powers.