Figured I'd update my original post in case anyone out there is tackling a similar project -
I went with my original plan, ran 10-2 Romex from a 20 amp GFCI breaker at the house panel to the outside, then changed over to #10 stranded wire in conduit to the detached garage, then changed to 12-2 Romex inside the detached garage. I'm glad I went with 10 wire because I didn't want more than minimal voltage drop. Where it entered the building I used a heavier duty single pole switch as the disconnect as code requires and then installed 4-foot T8 lights, a ceiling fan, and 8 receptacles, including one for the garage door opener and one for an overhead cord reel. The lights are great and work very well even in very cold NY temperatures. Haven't had any problems with dimming or tripping the breaker but I rarely come anywhere close to putting the circuit to the test, mostly running a little space heater to take the chill out and maybe occasional use of a compound miter saw or bench grinder. There's not a lot of simultaneous device use other than lights and one plug at a time so the single circuit suits my needs. My biggest complaint is the T8 ballasts destroy the music coming out of my FM radio but I knew that going into it so not a big deal. It's worth it because they're better in the cold and much quieter than the T12 high outputs I used to have in my attached garage years ago.
The building is basically a large storage shed that I also use for short periods of equipment maintenance so this works for me. I do agree with the other posters however, for anyone considering using a building like mine for extended periods during colder months, install a larger circuit with a small panel and if using electric heat install a 240 volt heater to take the chill out faster. I installed a Dayton model in my insulated attached garage because I still change my own oil and that thing raises the temperature about one degree per minute during even the coldest months of the year. It also makes my electric meter spin three times faster when it's running.
Thanks for the advice. Lots of good information on this site.