Back up, wipe, instal iOS, restore your backup.
Figuring out exactly what happened with a particular device is difficult and expensive with direct access to your devices, tooling, and time. With no access and remote discussions, determining if there are issues is based on evidence gathered, and that evidence needs to be pretty specific. Coincidences and gaslighting and other shenanigans is a thing that targeted folks do encounter.
What to do? Malware persisting past a backup-wipe-install-restore is quite rare. Problems with credentials, far less rare. Common, even.
There are folks that do have suggestions here for those experiencing problems with former partners, including Eva Galperin of EFF and the Badass Army non-profit folks.
Back up, wipe, reload iOS, and restore. Enable two-factor authentication, select unique generated passwords, upgraded passwords to your mail server accounts associated with password recovery, use a password manager, upgrade you security questions, establishing a trusted telephone number, and—if you’re particularly concerned about the current contents of your iPhone—a backup, wipe, reinstall, and restoration, as persisting over a restore is quite difficult.
Apps installed from another Apple ID won’t re-install without that Apple ID password, and you’ll get diagnostics there. I’d avoid re-loading or keeping add-on VPN clients, add-on security apps, and related. (Or set the iPhone up as new, and only load what you want and need.)
Most folks with iPhones and related security problems tend to get in trouble with phishing, and with passwords, and not so often with iPhone exploits. Which is where two-factor authentication and password upgrades helps. And the backup and wiping and reloading gets you a current backup, and gets your iPhone context reset on the off chance there’s malware or shenanigans here.
