Thanks for all the comments.

Actually, the LH flight departs at 1230. I went back and looked at the recent history of DL14 arrival times, and generally it is on time 80-90% of the time. However, I don't I want to be stressed and worried about after landing will we be at a gate, or remote stand, passport control, luggage , changing terminals, etc, and then missing the flight. I also looked, Delta will not interline the bags.

We had a friend who spoke highly of a visit to Erfurt, and its only a couple of hours by ICE train from FRA, and then a couple of hours to Dresden, so we thought it would be a good overnight stopover point to see its old town, and their Christmas Market, and avoid the stress associated with all the potential issue with changing airlines in FRA. I guess we will probably choose the 1241 ICE train, which should be a comfortable transfer (hopefully)