Since Reunion 12 is a legacy app, no further updates will be released to fix the compatibility issues between macOS Ventura and Reunion 12.
In case you're curious, Reunion 12 was supported through many different macOS upgrades. It went from a 32-bit app to a 64-bit app so it would run on macOS Catalina. (This took many months of coding.) A few updates were released to fix issues with the releases of macOS Big Sur and Monterey. Shortly after Reunion 13 was released, only one minor update was released to fix an issue in Reunion 12. Since that time, our focus has been moved solely to supporting Reunion 13.
Reunion 13 runs fine on macOS Ventura.
If you would like to check out Reunion 13, please see this web page...
https://leisterpro.com/doc/v-13/upgrade.php
A listing of new features added in Reunion 13 can be found here...
https://www.leisterpro.com/doc/v-13/...13features.php
I'm sorry I don't have better news for you regarding Reunion 12.
However, I hope you find this information helpful in moving foward