The UA RCC seems to get generally nice reviews here, plenty of posts to look through I believe?

The LH forum might yield some information re: LH lounges.

I have this list, but it might be dated?

FRA LH Lounges
FRA Senator lounges :
* Term A, level 3, gate A51
* Term A, level 2, gate A25
* Term B, level 3
FRA Business lounges:
* Terminal A, level 2, gate A26
* Terminal A, level 3, gate A26
* Terminal B, gate B24
* Terminal B, gate B44