Stuttgart to FRA

Check yourself with specific data: https://int.bahn.de/en
Station name is "Frankfurt am Main Flughafen Fernbahnhof".

Be aware that you need to be minimum 120 minutes before departure at the airport.

With the DB now, this is a risk given the potential delays & cancellations taking place even in the early morning. You need to be at FRA at 6:30am for a transatlantic flight. I recommend staying at a hotel by FRA.

There are hotels IN the Frankfurt airport, I believe a Sheraton and a Marriott. It's very nice just to get up and walk (inside) to your airport gate.
Safe travels!

I would not take a train that far for an early morning flight. Stay in Frankfurt or at the airport over night. For a flight to the US, recommended time to be there is 3 hours before your flight.
Check Hilton Garden Inn, Hilton, Sheraton or Marriott for airport hotels. Frankfurt is only 13 min. away by train if you want to stay in the city. A central hotel works well as the S-8 and S-9 go directly to the airport Regional station. Located under terminal 1, all you have to do then, is walk upstairs to check in.

Thanks all, I just didn’t want to add another travel day and lose last night in Stuttgart.

Thanks all, I just didn’t want to add another travel day and lose last night in Stuttgart.

This of course is up to you but now that you have to be at FRA at 6:30 AM due to your flight change, I would advise staying at or near FRA or change your flight.

Since your airline made the change, you shouldn’t be charged for changing it to a schedule that works best for you.

Ok I now have a hotel night before flight. Should I buy ticket from Stuttgart to FRA that includes city ticket? Would that work, a few hours after we check into airport hotel, to get to Frankfurt Christmas market for the evening?

The City ticket only gets you one way to your final destination on local transport. It is not a local transport day-pass.