Multiplayer almost exclusively. As soon as I got the basics down in the licences, the GT Sport AI became boring to race. Even on professional mode, it's just far too slow. Becoming more proficient at racing only exacerbated the gap and made the AI frustrating to race against.
Every time I do a single player custom race to practice for FIA races and get a gauge for race time, I set up a 1v1 simply so I can disappear off into the distance without interacting with it. Yet in the back of my head, I know that I'm probably going to have to lap it at some point. I'm then hoping it's going to happen in a place where it's easy to zoom straight past it because of how weird it acts when getting lapped, and how it likes to brake hard in places where a human would brake lightly, lift or even in some cases just keep their foot down.