Use the standard text sizes in iOS
I use the small font size in my device settings, but I have never seen text as small as in Teams for iOS.
Please use standard text sizes throughout the app. This will mean that the text size that your users see will be the size they expect to see, rather than whatever makes it easier to design overly complex UI with.
I understand this will be a large rewrite, that’s okay, the sooner you do this, the better.
Please refer to the Human Interface Guidelines provided by Apple: https://developer.apple.com/design/human-interface-guidelines/ios/visual-design/typography/
Microsoft also has some advice on the importance of text size: https://docs.microsoft.com/en-us/windows/uwp/design/accessibility/accessible-text-requirements#text-font-size-and-scale