I don't have any recommendation for Frankfurt; it is mostly known for Äppelwoi (which can be quite terrible imo). But I'll try to give some more general recommendations for German beers (although you seem to know your way around anyways).
I don't know how easily available these beers are where you come from, but if you like Weißbier then you need to have
Schneider Weiße (TAP 7 is the regular one, TAP 6 "Aventinus" is a dark Weißbierbock, TAP 5 is a Weißbier-IPA hybrid) and - a discovery I only recently made myself -
Unertl Weißbier. If you can find it anywhere, Riegele's "
Augsburger Herrenpils" and Schönramer's Pils are the best types of German Pils that I know (in particular the latter, but I haven't been able to source it anywhere besides online shops). Riegele also has a delightful series of "modern craft beers", in particular
Simco 3 (a Pale Ale showcasing Simcoe hops)
, Robustus 6 (a porter
) and
Augustus 8 (a dark Weißbierbock
).
Ahhh this takes me back 4 months when we traveled from Munich to Berlin to Köln to Frankfurt. This seemed odd to me, but when we asked for a Märzen, we were almost laughed at! Could've been because it was during Oktoberfest, but found it odd. Especially here in the states where craft breweries go with Märzen as their Oktoberfest over the lighter festbier that's the staple in Munich. When in Frankfurt we couldn't find too many Frankfurt local beers, so we went mainly with the festbier. Have a great trip man!
The landscape of beer is rather limited in Germany, unfortunately. The offer in restaurants and pubs is mostly dominated by a few larger breweries, particularly in Munich. Regarding beer styles, in Munich you can find Helles, Weißbier, Dunkles Weißbier, sometimes Dunkles (which tends to be a Helles which has been coloured), sometimes Pils. During the right season, you can get a Bock in some places. I don't recall ever drinking a Märzen anywhere in Munich.
In spring, some friends invited us to Franconia for a beer tour. I'm looking forward to that, I think they have a lot more intact smaller breweries.