Just noticed your trip is a year away ... so you do have that all important 180 day mark.
The Iluminations Grand Gathering has to be booked through the Grand Gathering (Group?) Department - it is only offered on certain days of the week and is only available to you if there are 8 or more people (aged 3+) in your party - if this is a must do, it will eliminate some of your options.
That said - I would definitely do it - the view for Illumination alone is worth it! The deserts they served were yummy too - as was the dinner itself. We were an older group (aged 12 to 66) but still found the "show" part of the dinner amusing - the kids in the room really seemed to get into it. At the end, Minnie and Goofy made an appearance - which is another plus.
Once you know which day your Illuminations Dinner is on, you can have some flexibility with the others. Oh, how many days do you plan on being at Epcot? If only one, is the Askerhaus Princess meal a must do? (I haven't been, but it seems like you have lots of other princess options, so I would choose the Grand Gathering over the princesses at Askerhaus if something has to go.)
Just noticed you also have the Garden Grill and Coral Reef, which are also at Epcot. I'm a one park a day person ... so it seems strange to me to have so many choices for one Park ... but if you park hop, I guess it works out fine!