With regard to the original question about RF2, while it has a great variety of cars and tracks, and an excellent physics engine, Le Mans Unlimited is its successor, with even better physics, a superior UI, and VR experience. AMS2 solves the track/car issue.
I drive mostly SP LMU and Automobilista 2 (AMS2) these days.
LMU is focused on multi-class endurance racing and demands a fairly high level of driving knowledge. However, the reward is that you are driving probably the most accurate consumer level driving sim. It has far fewer cars and tracks, but what it has are modeled to an extremely high level. In the last year, LMU has much improved its UI and VR driving experience. As @NEVO mentioned, multiplayer racing is hosted by RaceControl. It has a co-op mode, but I believe is limited to 3 drivers racing on their own at the time they choose.
AMS2 out of the box has a much wider variety of cars, including modern, vintage era, and amateur types to keep the racing accessible and enjoyable for more skill levels. AMS2 has always been thought of as the best VR experience, although I think that LMU has closed the gap. It also includes many of the tracks and endurance racing experience as LMU. In both sims, some may require additional purchase.
AMS2 makes it easy to create championships with built-in templates. Want to setup a vintage F1 or a modern IMSA championship, it’s easy to do with a few clicks.
AMS2 has infinitely more mods and skins available, and has an easy to use mod manager. Basically, drop a zip file in a folder and click one button. For instance, there is an excellent Aussie V8 Supercars mod. I thought that it would be awesome to host a MUDSPIKE race in those cars at Bathurst. Or an IndyCar race at Long Beach, IMSA race at Sebring, or vintage Group C cars at Spa. All available in AMS2.
Similar to LMU and RaceControl, AMS2 is supported by online race hosting and ranking service, Low Fuel Motorsports (LFM), making it easy for drivers to continue racing outside of Mudspike.
You can host private group multiplayer in AMS2 (outside of LFM). From this excellent reddit post, it is best hosted by one driver setting up the race and hosting while driving. There is a dedicated server, but it is more of a matching service than a server. The first driver to join becomes the host, regardless of where the ded server is located. If that user is distant from most of the drivers, then all will suffer. But with that in mind, setting up a PvP race or race series in AMS2 should be fairly easy. BTW, if only a few drivers show up, you can fill the grid with AI. Weather management is fairly easy.
LMU has WEC licensing and AMS2 has IMSA licensing, FWIW.
Regarding AC, you can do pretty much everything (and more) that AMS2 can do, but you need a complex management tool and graphic enhancement add-on. The original UI is outdated and confusing for newbs and setting up championships requires long hours and has a steep learning curve. Virtually any kind of racing and off track driving experience can be had in AC. Just be prepared to spend a massive amount of time modding and tweaking. And VR is a whole other can of worms.