I've been trying to get my wife into overlanding with me, but she isn't a fan of waking up with a sore back from sleeping on a thin sleeping pad. I see several out there on the order of 4 inches or so, but very few have reviews. I'd like to use this on the ground at first, and eventually maybe transition to a roof-top tent.
Are you looking for a foam type or blow up?
I have the Big Agnes Q-Core SLX and it is quite plush. It's a 3.5in air pad, I can sleep on my side with no problem and it's held up well.
I also have had good experiences with Big Agnes pads. I'd say that blow-up options are more well-suited to people would want a thicker layer for side-sleeping, etc. Air Core models are great if you want a cheaper option.
Klymit is the other big brand that was a frontrunner for me. Try out their more interesting designs in person first, if you can, though.
If you want true comfort, the exped megamat; I had a worst mattress in college. Definitely not a backpacker but great for carcamping/overlanding. They make a "duo" which is slightly more narrow than a queen mattress. I opted for two of the standard and use a mat coupler.
If space isn’t a major issue I would HIGHLY recommend the foam ‘matresses’ they sell for kids sleepovers. They’re bulky but are so worth it. I sleep with one over my cot when camping for 7 night stints and will never go back.
I bought the Cozzzi Trifold Foam Folding Mattress off of Amazon when it was on sale for $60 about a year ago. It is 4 inches thick and the 75 X 25 size option works great. FYI I’m 5 foot 7 and it’s perfect with space to spare.
I’ve since had a few coworkers of different heights and weights purchase similar styles from Costco and Target and they all love them.
I have the Exped DownMat XP 9 and it is fantastic. It's not the lightest but I will carry the extra weight because it is super comfortable and it keeps me warm. I sleep on my side and I don't feel the ground even when I move around. It has a built in hand pump too. I love it!
I haven’t see it mentioned yet so I thought I would offer a personal recommendation for the Thermarest Mondo King 3D. I have two and they have served the exact purpose that you asked for (creating a camping comfort for my wife). Now I won’t go back to anything else when car camping myself.
After just getting back from A two week camping trip through out Northern California I would highly recommend NEMO Tensor Insulated Sleeping Pad. This is for side sleepers and it will keep you off the ground.
REI has this item in stock.
I use the Big Agnes air core ultra. It's not insulated and can be loud when you roll around, but neither of those is an issue for me. It's not the lightest out there, but It's very comfortable, I usually sleep on my side and never feel the ground. It weighs 22 oz for the 20"X78" size. It's 3.5" thick. I've used it for several years and never had any problems with it. If it dies, I'll probably look for something lighter.