He can be prosecuted depending on the statute of limitations. I don't know whether the statute of limitations "tolls", meaning that it doesn't run during the period of his presidency. It would make sense to me that it would. Statutes of limitation are intended to preclude undue delay by the prosecution and also to allow a defendant the opportunity to put up a defense while the evidence is still fresh. The first purpose at least weighs in favor of tolling the statute since the prosecutors are not constitutionally permitted to prosecute while he's president.
Barr would not be AG once he's out of office. Could a Republican AG in another administration do something to bury it? Yes, but he or she would be risking his neck if he instructed U.S. Attorneys not to investigate or prosecute serious crimes.
It can also be prosecuted at the state level. The Attorney General could not do anything about that. Also, the next President's pardon would not reach state crimes since only federal crimes can be pardoned. I only skimmed this old politico piece but it said something about federal crimes and statutes of limitation so it's possibly useful!
https://www.politico.com/magazine/story/2019/06/06/how-trump-could-be-prosecuted-after-the-white-house-227050