You could try different routing. I hate that in the winter timetable LH only has the 16:45 out of BOS... In the summer they have three flights between BOS and FRA/MUC. I fly that route quite frequently and that 5:30 arrival is not a great way to start a day.
Try via JFK or IAD. There's bound to be a better flight. For example, LH9253 (operated by UA) leaves IAD for FRA at 21:40.
This won't solve your problem (still have plenty of time in FRA) but at least you get a good night's sleep on the plane. Of course if you can fall asleep at 5pm then it's really a non-issue.