If I was in your position I would:
- check in online
- take the RE at 0651, with a plan to connect to your booked ICE at 0755 in Köln.
- if you miss the connection, then take the ICE at 0827.
In general, I would aim to take the train before the last possible one to make your flight, unless you have a flexible ticket at no time pressure. Long Distance Trains in Germany are 75% on-time (defined as arriving within 6 minutes of the scheduled time).