I would never recommend Thrasymachus for an intermediate. I think it's too hard (and schematic) for a beginner and too little for an intermediate. Anyway it is too schematic for both beginners and intermediates. Perhaps what you need is a grammar for consultation and a few original texts for refreshing your greek. There are on-line some grammars and many texts for practicing. For instance, just to begin somewhere
I'm sure other members know better sources and references.
I am writing in Ancient Greek not because I know Greek well, but because I hope that it will improve my fluency in reading. I got the idea for this signature from Markos over on the Agora forum here at Textkit. Every suggestion as to improve my Greek will be welcome.