the 'sights' in DBX = the tower, the beach... 24 hours will be all you need.

The rail trip across a bit of Europe would / could be fantastic, depending on your routing and stopovers. I would make the last leg (into FRA) on the ICE train - coming from the west, you will probably pick it up in Aachen or even Brussels, unles you wish to head north first, in which case you can get it from Amsterdam.

The great debate on stopovers will be: do you want to travel and sleep during the night, to maximise daylight city-seeing (and miss the scenery from the train)? or visa versa, watch the World (Europe) go by in daylight and then enjoy city nightlife??? Both work well.

As far as the flights go, it all looks great (I'm thinking CX in F) tempered with a bit of a slog (that long Y sector(s))