I believe lowering the gear can be done. Like people here said, you have two options:
1. change to mid or long cage RD to accommodate a bigger cassette up to 34 or even 36 T cog.

2. change your crank to 53/39 or even 50/34, but you said the FD can't be lowered. If that's the case, that's must be the FD hanger too high. You can saw the FD hanger off, buy a band on FD, which you can put wherever you want on the seat post. But probably you don't need to saw the thing off, just need to enlarge the slot on the hanger a little bit so the FD can be lowered.

If you think this is too much work (which I think is), pay the $250 for the mechanic or change a bike (vector p20 seems a good one).