Zone 8 grandma, nice solution that fits well with the surroundings. It looks very inviting. Chambma, I've not tried it yet, but have heard that straight 5% white vinegar (a one-gallon jug at the grocery store is really inexpensive) will take care of weeds in a drive or similar location. Just spray it on and they should "burn" up in short order. Just be sure you don't use this in a garden or lawn, as it can change the pH of the soil, making it unsuitable for any plants you do want to grow there. I know what you mean about the round gravel. That was in our drive when we bought the property (and migrated everywhere else!) and we finally got rid of what was left and replaced with crushed bluestone. I posted it on Freecycle and someone came and took it away to use under a shed he was building.

You might try wild thyme... aka "mother of thyme". The Latin name is "Thymus serpyllum". It is a hardy ground cover, aromatic, and flowers profusely once established, usually with rose-coloured blooms. Great bee plant, aggressive enough not to be overwhelmed by weeds and eventually you will have a fragrant, springy carpet underfoot. Can be grown from seed or you can buy starter plants from a good nursery. I suggest you rid the area of other weeds - two or three times if they are persistant - either by using a product like Round-up or by torching. Then plant with the wild thyme and after that all you need to do is water until established. Among the most care-free of plants.

If the current and future gravel is crushed and not rounded, then I would think the 4 inches would be adequate. If the stones are rounded, then no depth is adequate. I have crushed stone drive. The stones are pea gravel sized but they are faceted and not round. Once they are locked into place, they don't move around. Mine does not have a border, either.

Those delosperma look great. I hope mine come up strong this spring, they seem to be a favorite of the rabbits and struggling as a result. There are some great ideas in this post, but I can't get past the image in my head of a nice perennial edging that's got a ton of weeds and grass sprouts growing in all over the place. Must be the scars from a childhood spent weeding bluegrass out of creeping phlox.... and that was just a 3 foot patch, not 700 feet of driveway edging. Sure you don't want to just line the drive with a couple dozen crapemyrtles or some other shrub that can be easily mown around?