Actually, “offline” products can still be purchased from the official hub: ESA Copernicus data access - Long Term Archive and its drawbacks
But there are also other portals which allow the download of all products in batch mode, for example PEPS.
Please have a look at this list of sources: https://krstn.eu/awesome-Sentinel/
It also provides different methods for atmospheric correction.
Maybe the products from the Google storage are slightly different from the officially provided ones? You could try to download one of the products which failed in sen2cor from another source and compare if the problem persists.
About batch processing of sen2cor: