When I went the first time, I had the fog lifting experience and the best photos. But as tickets sales are final it does not really matter whether one can know in advance (by following the weather forecast to avoid the worst rain). A bigger problem now is that you have to follow the route in one direction and they are not going to let everyone wait at the famous viewpoint "Watchman's Hut" for fog to lift. Under the current rules I might buy one afternoon ticket, spend the night in Aguas Calientes, and then have a morning ticket the next day (afternoon and early times sell out the slowest because so many people come from Cusco and cannot arrive untill mid morning at the earliest.) But you will still need to buy weeks in advance.

May is also a great month overall for visiting. Yes the terraces should not yet have turned brown as they might be at the end of the dry season.

If I was only going once, that would be the month I would pick. But if I had nothing to do in December I might go if I could scrape the itinerary together.

Bear in mind that Peruvian school calendar is going to have students off from mid-December on, but it's considered "summer" so beach destinations are most popular. Still you will probably find it easier to visit as early as possible in December. Good luck with your decision.

Personally, I would have no compulsion to visit Rainbow Mountain.