Feeling crampy and uncomfortable today. I had a bad night with reflux which I've never had, alternating with terrible back and pelvic pain. What the hell is my baby doing in there?

I don't know what to make of any of this. It feels like a warm up, but with DS it was a week of warm up and it did my head in. I don't think I could take that again.

I am also feeling unprepared. I haven't even had the birth plan meeting yet.

Urgh. Suddenly I don't want to labour!
The whole back and forth of "I want to have this baby!" vs. "I don't want to do labour again!" can be kind of miserable at the end. I remember it pretty well. I've experienced it more and more with my later babies, and it stinks. My only advice is to handle it kind of like you handle labour - just take one step at a time. If you're feeling crampy and exhausted and emotional, do your best to find a time to rest, take a bath or shower, a nap, put on a movie for the kids and put your feet up for a bit - whatever it takes. Just deal with today, right now. Don't let your mind wander to how long it might last or how much of it you might have to take. Just like in labour, it will make things worse.
Your body is definitely getting ready to have this baby, and even though it might feel like you'll never be done with this pregnancy, before you know it, you'll be looking at it from the other side, nursing your baby, and thinking "Wow, that actually went really fast, can't believe it's done already!"
For the back/pelvic pain, maybe some pelvic rocking/cat stretches might help? On your hands and knees, take a deep breath and pull your belly and back up, hold, and then slowly release back to a neutral spine. Repeat. You can do 20 repetitions or so at various times throughout the day. It should help stretch the sore muscles and strengthen them, and help get baby in a good position.
All the best in these last few weeks!

About labour positions - I have chosen to be semi-reclining in the bath for all my home births. (For DS1, when I had to move because he was stuck, I ended up delivering on my back in bed with my knees cranked up to my ears.) I've tried the hands and knees thing also and found it impossible for me. Squatting also is not an option for me - I just don't like it at all. But I think the idea is that you should be free to assume the position that works for you, and that usually that's the position that is best for you.