First, skip the 12 gauge for turkeys altogether. Instead, buy tungsten-based heavy ammo which will outshoot any lead in a 12 gauge.
For a good 20 gauge semi, there are a number of options, many based simply on what price range/social standing you wish to inhabit. Here are some that will work with your criteria of being reliable and easy maintenance:
Tristar Viper G2
Weatherby SA-08
Stoeger 3020
Franchi Affinity
Winchester SX3
Benelli, various models
Beretta 30* or 39*, various models
Of those, I think that the Franchi Affinity is currently the best if you desire high-quality with low and easy maintenance. It is an inertia gun with the spring under the forend.
If you need a gasser to relieve recoil for high-numbers shooting, then either of the first two get you there cheaply, the Winchester in the middle but heavier, while the Beretta gets you there expensively.
