Hey Feral.... I'm not going to quash your idea. Its a good one. But I hope you seriously consider the logistics of hauling your carcass out into the hills. I'm guessing the quad won't do the job. It'll take a righteous 4wd pickup. And carving the grave out of a mountainside isn't kids play. Worse still, some locales require a permit to do that. Edward Abbey confused the BLM for years because they thought they knew where his followers were going to inter his remains. Fooled them, too.
In a story like yours, back in 2005 we were at a jeep outing in Gunnison, CO. We took a trail up northwest of the town known only to us as the "Powerline". It was a long rock uphill, maybe a mile or so long. Loose rock and occasional steps, but none over maybe 18", most smaller. When we got to the top we were pretty strung out. So I pulled over to wait for my buddies scattered off down the hill. It was apparent it would take a while, so I took a walk to see the view. Behind a big pine tree was a grave, much like the one you described. I just found my photos of it. The view was great, particularly down toward Gunnison. Nothing short of spectacular. And no one on the trail would have seen it unless they stopped.
But a closer look at the cross stuck in the ground revealed the tag nailed to the cross. It was a dog tag. No, not military, but dog as in pooch. Some guy must have really loved his dog, and did right by him after he passed. I keep thinking that guys dog has a better resting place than I'm destined to get!
If you don't mind them cremating your remains, planting you where you choose is pretty easy and only one or two people can manage it on quads. If you require a full casket, its going to require that 8' bed pickup. And caskets need to be buried deep underground. Mountains are made of rock, usually. There might be a little surface sand and soil, but once you get down a bit its pick and shovel work. I'm guessing the standard 6' down was cheated on a bit in the past, conditions permitting.
What you say you want isn't unreasonable. But you need to give it some thought and talk it over with your wife. She's the one the burden will be on to follow through with it. If you've got any friends who would help, it'd be a lot easier on her. It sounds gruesome to some people, but I agree it would be better than being planted in a subdivision of other graves.