Hi,
There’s currently a known issue specifically with Reaper’s command line interface (which FMOD Studio uses to integrate with Reaper projects) where the usage of some VSTs causes the render process to crash. While we have reported this crash on to Cockos on their official forums, this is unfortunately not an issue we can address. There are a few workarounds listed in the known issue post, but you can also try using VST2 versions of any VST3 plugins, and disabling the offending plugins after bouncing audio in place.
To confirm that this is in fact the issue you’re running into, I’d recommend trying to use the Reaper CLI with the -renderproject argument to render your central Reaper project and see whether you get the same issue.