I recognize this thread is a few months old and OP's trip would've already come and gone, but this thread came up for me in preparing to get the Epcot fireworks cruise for my own trip-- so I'm sharing my own observations and results in case it helps others.
My target date to book an Epcot cruise for was either 1/5 or 1/7, and last night at midnight I successfully got 1/5, when that date opened up for me at 60+2 tied to the window for my on-site resort reservation. This morning I noticed that date appeared to be sold out. No idea if I got the last one or not.
This seems to have been in line with my keeping an eye on other late December /early January dates and as I had another booking window open up to 60+3 earlier in the week. Anecdotally, these seem to sell out roughly around 60, 60+1, or 60+2. Maybe it becomes more available at other times of the year, but OP's original observation that these seem to sell out by the 60 day mark seems to be fairly consistent. Note: I didn't take a look at MK fireworks cruises, this was for Epcot only.
Hope this helps someone! I'd definitely recommend prioritizing this at midnight on a day when you can book at least a little ahead of the 60 day mark as an on-site guest. Off-site guests might need to be extremely extremely lucky to book this.