Train from Wurzburg to Frankfurt

Flughafen means airport in German, so yes the train station Frankfurt (M) Flughafen is at the airport. Walk through to the terminals. Time might depend on at which terminal your airline is.

We're traveling on Delta, which looks like Terminal 2.

You need to get there a bit earlier then. If it was me, I would certainly plan on arriving at the airport 3 hours early. One always has to add in a cushion of time to allow for train delays.

Allow at least 3 hours at the airport. Last time there security took a loooong time.

I've come in to the airport from Karlsruhe and from Boppard, but both times the flight was after noon.

I usually get to the airport 2-2½ hours before flight time, but my flights leave from Terminal 1, and I have only carryon. Only once did I get there 3 hours early. That was for Northwest (now Delta?) from Terminal 2. It was my first flight from Europe in 10 years, and NWA said to be there three hours early. By the time I got to Terminal 2 is was a little less than 3 hours early, and there was a huge line (apparently everyone else got there 3 hours early). There were only two people checking in an entire 747, and whenever a 1st class passenger showed up, one people took care of him, and there was only one person for economy. It took over two hours waiting in line before I could check in. Almost no one arrived behind me. I could have come an hour before time and still checked in almost as soon.

I see there is an ICE leaving Würzburg at 6:55, getting to the Fernbahnhof at 8:22. I think that is enough time, but there is another ICE at 6:26, arriving 7:55. With advance purchase early enough the fare for 2 would be 29€. That's a non-refundable Savings Fare ticket, but if you missed that train, loosing the ticket would not be your worst problem.

Or, you could stay in Mainz or Frankfurt and sleep a little later.

Many thanks, this is very helpful!