Is uninstalling apps on iOS thorough?
Yes, app uninstallation on iOS is very thorough. The app is in a normal circumstance not able to leave behind executable bits and pieces to infiltrate your phone, spend resources or otherwise change the behavior of the phone.
Note that the app might leave data behind in the iCloud, in your Keychain and other places - but it is data, not executables.
The reason I write &quot;normal circumstances&quot; is that you're considering app made by national states for the express purpose of spying. In that case, it is possible that the app exploits some kind of vulnerability in iOS that allows it to alter your system permanently. This would have to be done without somehow alerting Apple to it in their reviews. I wouldn't consider it likely for almost any app though.