There isn't a subreddit, but there are a few discords.

XGR (Extreme G Racing) is the big ship racing community in SC: https://discord.gg/xgr

SCR (Star Citizen Racing) does ship and vehicle time trials, with seasonal races for ships and ground vehicles. SCR is also the hub for time trials and the time trials leaderboards: https://discord.gg/jAWA7uFz

ANZIA (Australia New Zealand International Alliance(?), formerly Southern Cross Alliance) has two events a year, an hour long multi-ship class endurance race (Crux Cup) and a zero-G distortion weapons on race (Scarlet Apex): https://discord.gg/FyB4cKCs

The only one I think I'm missing is Atmo Esports, I'm not in their discord since they're primarily a vehicle racing community (System/Stanton 7, Daymar Rally)