Hi Christopher,
Thanks for the question.
Considering that you have already tried uninstalling the app, restarting your Iphone, reinstalling from app store and cleared cached data when prompted during reinstallation, the next step you could take is resetting app permissions - Go to settings > Copilot app > Permissions. Also ensure network access and background app refresh are enabled.
Sharing a resource that might be helpful for you to gain clarity: https://learn.microsoft.com/en-us/answers/questions/5664726/why-microsoft-copilot-app-is-sometimes-down-on-ios
Hope this was helpful! Please let me know if you have further queries, in the comments.