Do you have the same issue if you follow the OP’s steps to reproduce it?
Yes. Just did the same, reverted to SU9 and CTD on loading.
However, I was using the Steam process as specified here: https://forums.flightsimulator.com/t/read-first-welcome-to-sim-update-10-beta/507612 but the end result was the same. CTD halfway through loading the sim.
Provide extra information to complete the original description of the issue:
It looks to me like the update check after reversion to SU9 isn’t working, and it leaves SU10 specific content in place which I think is what is causing the CTD on loading. If their update check worked properly and redownloaded the SU9 content then you would not have to do a full reinstall, which is just a ridiculous thing to have to do.
Why can’t they fix this? They have a high priority beta underway, but people can’t easily switch between them.
I reverted back (back actually!) to SU10 Beta and all is working ok.
That would probably be because the content matches the executable…