Update again...
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...hello from Laguna Seca. I've had a couple hours with few issues. Before getting to Laguna Seca, I had a brutal crash to desktop upon completing the Cadillac hypercar segment I mentioned in my previous post. Fortunately, when I restarted the game, it simply let me retry from the same segment - I didn't have to re-do the entire intro or jump back in the Corvette E-Ray.
After that, I spent a while adjusting settings. Benchmarked, graphics settings adjusted, sticking to controller before I jump into the wheel settings.
I've had 11 laps of problem-free driving in Test Drive mode at Laguna Seca in the evening, and given my car choice, I had some big Gran Turismo 2 vibes.
(the livery probably gave it away that I love this game)
Switching the event to Circuit Race, however, I immediately encountered an aggravating bug: Steering wouldn't work anymore! I could accelerate, brake, shift, but not turn the wheel. I believe I've seen someone else on the forums explain having this problem. This game has some growing pains to get through, it seems...
I had a bit of a tech support lightbulb moment, so I immediately tried the Administrator Mode method after I updated Gaming Services. Not sure why, I just had a hunch that trying your fix might work
now and lo and behold, it did. We're not totally out the woods yet but Forza Motorsport has been promoted to playable. It's something.