I have done the J class to FRA and then hop a flight to TXL several times.

I missed one and had to pay Lufthansa a 70 Euro change fee. The good news is that there are hourly flights from FRA to TXL. I am not sure I would book J class, it is a short hop and European Biz Class is not really much. You can get a good economy fare with a seat assignment and one free checked bag on Lufthansa for under 100 Euros.

I had given myself over 2 hours, but the plane sat in ATL for 2 hours. Just know that your risk exposure with Lufthansa will be about 70Eu per person if you miss. The flights are usually between 70-100 Euros one way in economy anyway.

I would hate to wait 3 hours, but have started doing that. I tell myself you can't get into Berlin too early anyway as the hotel might not be ready.

As for the return, I would not want to be on that train that long. It will be a long train ride. Like close to 11 hours. I would rather fly into FRA, then spend the day in Mainz or Weisbaden roaming around.

Hope this helps.