If there’s a very specific place or room category you need, book as close to a year out as possible, that goes double for holidays.
We needed a 2 bedroom villa for a trip once and certain members in the group kept dragging their feet because they didn’t believe me about booking it a year out. This was for a September trip a few years back when that was still the off season. At 9 months out, we had only two hotels left with availability to choose from. The same members of the group who held us up were now complaining why couldn’t they stay at one of the other hotels. I was like because you wouldn’t listen to me when I told you we needed to book this 2 months ago.
On the other hand, if you don’t really care where you stay, you can usually find last minute deals a few months out. However, with Disney’s cancellation policy, there’s no harm in booking early and canceling or modifying later on if you know your dates.
I’d definitely go ahead and make dinner reservations at 6 months. It’s so much easier than scrambling to get them later and like with the room, you can always cancel or modify if needed as it gets closer.
Make sure whatever you do, you have tickets and your room before 60 days, so you can set up your FP. Again, not impossible to get later on, but much easier to do in advance.
I also agree with avoiding Christmas / New Years and going during the beginning of December or the very end of January beginning of February. Don’t try for beginning or middle of January. That time is almost as bad with holiday crowds lingering and the races. Also, those tour groups you mentioned come in January/ February and July not March. March is beginning of spring break crowds. If you do go for Christmas make sure you know what you are getting into. Read some blogs, look up pics and YouTube posts to see the crowd levels and other general advice.