Who gave boot jack smith subpoena power, anyway?
He doesn’t have subpoena power; he had to get a judge to approve all the subpoenas he requested.
The actual spying or gathering of information on all the Republicans was probably legal, Jack Smith went to court and got subpoenas for everything he did.
What should be investigated and is probably illegal is the reason for the investigation to begin with, usually there is some indication of a crime before an investigation can begin and subpoenas can be requested.
The illegality goes higher up the food chain, who authorized the investigation and what was their reasoning, that is what needs to be uncovered, investigated and prosecuted.