I'm not sure that switching from iOS to GrapheneOS or stock Android will solve the issue with your partner. I suspect the person may have installed spyware on your device. Spyware that constantly accesses your microphone, camera, GPS, storage, contacts, and SMS messages may be cross-platform (iOS, Android, and of course GrapheneOS), so if it was installed on your iPhone, it could also be installed on your Pixel running either stock OS or GrapheneOS. If this is the case, I recommend reviewing your installed apps and their permissions, then removing any unknown software (but, please, don't touch system packages).
There are some useful improvements like duress passwords, 2FA passwords, and scrambled PINs - nice to have, but they don't really address the root cause.
Or maybe the person is simply monitoring what you post on Instagram stories to track your location. In any case, I believe you need to improve your online hygiene rather than just installing GrapheneOS.
As for your question, when an app crashes, you should see a notification like XDrip+ tried to perform DCL via storage, which means the app attempted to bypass some of the OS hardening settings. Tap the notification, and you'll be redirected to Settings where you can switch from Default (Restricted) to Allowed, relaxing the security option only for that app.
If there is no such notification, then long-press the app icon, tap App info, and enable Exploit protection compatibility mode which will disable almost all security settings for that app.