The DL14 flight to Frankfurt generally arrives on time, but you really will have to handle this as separate tickets as LH and *A are in T1 and "everyone else" is in T2. So you will have to go through controls of passeports (often slow), collect luggage (also slow)/customs, take the Skytrain or shuttle to T1, and start from scratch at the LH counter. So 3h is the absolute minimum I would plan if you wish to make a separate ticket to LH, with the understanding that you're on your own if something goes wrong.

I doubt DL would interline, plus it would be risky as LH handles operations at T1 and an unreliable, strike-prone contractor handles T2 operations.

If you opt for the train, I would not take any train departing before 12h. You would have to catch the same shuttle to T1 where the train station is located.