My lounge of choice in FRA is the Panorama Lounge near A26 or if I need something warm to eat the business lounge directly one level above.

on longer layovers, first having something to eat upstairs, then relax in Panorama lounge.

in general both of them are usually not that busy and much more quite than the other ones.

probably since few people know about them and it is a long walk, often passing by the other lounge options.

Both have showers btw.