No need for a flexible ticket; if at any point the estimated delay on arrival is 20+ minutes, you can take any routing/train you want. Taking a screenshot of the app (where the 20+ min arrival delay is shown) might help in case the delay happens to disappear from the system but you're already on a different train. In my experience, once you mention your original routing was delayed, DB staff just nods and doesn't bother you any further.

BTW, there are currently construction works between Frankfurt Hbf and Frankfurt Airport, so 5-10 minute delays getting to/from the airport are more or less the norm at the moment. If you go up to the airport rail station from the platform, watch for signs directing you towards shuttle buses to Terminal 2 - this is a faster option than walking to T1 and then taking the SkyTrain to T2.