@LilSki
Hate to throw water on this thread, but I've noticed an issue that may include other (but not all) tracks, but Riverside is one of them that's causing issues for me.
With my current settings, I'm able to run >180FPS on this track alone in a Practice session, with both CSP+SOL on, as well as most of the other detail settings turned to maximum. Adding the 23 AI opponents drops that to ~40, without any noticeable "maxing out" of my hardware to be seen within either Task Manager's hardware monitor or MSI Afterburner's.
My assumption is that, since this track hasn't received an update since 2017, that CSP or Sol has done something in the meantime to "uncap" some limitation that you didn't have to worry about when optimizing your most recent build, and now that it's uncapped, it's somehow overloading things before it even tries to tax my system.
That said, my only indication that this is the case is the difference I see by simply disabling CSP and hopping back into the same AI-populated scenario, doubling my frame rate at the very least, which puts me about where I'd expect to be, as my system actually starts getting taxed at that point. For whatever reason, doing so makes the game not recognize input from my G29 once I hop into the actual driving, but obviously that's not your problem.
I can do this same sort of scenario at a load of other tracks without issue, including both Kunos and user-made content with their own major amounts of CSP integration. This isn't meant to be a bash at all, just an acknowledgement that it's been a little while and may need another look, seeing as it's one of your most "legacy" products.
If you need any further info from me, I'm happy to oblige, just didn't want to make this message any longer than it actually is. Thanks!