1) This is just in general but how do you guys plan out something this far in advance? Weather is so unpredictable, at least where I am (Denver). I did my first real cross country back in May, from here to Los Angeles and that worked out but it just seems like planning a cross country months in advance is ripe for having to change or cancel based on weather. Taking time off from work, etc, just to have things fall apart closer to the date of departure just seems like a real issue.
Flying is not for people who make a Plan A and stick to it no matter what. Those people will end up dead.
Flying is for people who make a Plan A, B, C, and D, think through possible issues with plans, and can come up with new plans "on the fly" when conditions change. I like to tell my passengers "Aviation is always an adventure, it's just not always the adventure you planned on." But, I have had some of my very best experiences when all the plans go out the window.
So, flexibility is key. Maybe plan a direct route, and two more routes that are offset north and south in the middle. On a longer trip, you can go well off route in the middle without affecting your total flight time by that much - On a thousand mile trip, you can go 100 miles off course in the middle and it only adds 10 miles to the trip. Also, look at places you might want to stop along the way in all of the above scenarios - Places with cheap fuel, food or courtesy cars, and maybe even local attractions that you're interested in so you can take a break during the hottest/bumpiest parts of the day and/or keep your spouse/pax happy.
Do NOT start looking at 10 day forecasts 10 days before the trip. They're wrong. Start looking at MOS maybe two days before the trip, and consider whether leaving early might be advisable. Never cancel a trip until the day of, and keep in mind that you can always go a day or two later/earlier. Once you get an instrument rating, the number of times you'll have to do that goes down.
FWIW, my first time at Gaston's I was not instrument rated and I did something similar to what you're considering - I flew from Wisconsin, and once the gathering was over continued to Houston to visit my sister.
@Kitch is absolutely correct that you'll learn a LOT on long XCs. A 182 is one of the best airplanes for taking a trip like this as a newer pilot (I was also flying a 182 on my first trip to Gaston's!) and spreading your wings a bit. Do it!!!
4) I've never landed on grass. Do I need to practice that a bunch before making this trip?
It's never a bad idea to grab an instructor with experience in something you haven't done the first time you do it.
However, you'll probably realize that landing on grass is a non-event, it's just... Different. Gaston's does have a few bumps in certain places that can give you a bit of a kick into the air or a bit of an unexpected yaw, but like any situation, you simply have to control the airplane and not let it control you. And they have improved it over the years. I wouldn't let the grass runway be the thing that stops you. If nothing else, land at BPK and there'll probably be an instructor there who would be happy to go with you for your first landing at 3M0.
5) What should I expect? Is it just people hanging out telling pilot stories of which I have very few lol, or is there more going on?
I mean, there's definitely other stuff going on - "Normal people" go to Gaston's too - But we're pilots, we're gonna talk about flying a bunch. Don't worry if you don't have a lot of stories yet, just listen to others' stories! Pilots are generally very interesting people, so discussion will often stray into other interesting hobbies, some of which you may share. I've had fun there every time regardless of my experience level at the time.