There are ~$30 suspension seat posts. THey are crap. I think Cane Creek still makes the excellent Thudbuster.
In the early 90s I used a Softride suspension stem on my MTB and I found, despite the drawacks, it was more effective than the rudimentary susp. forks of the day.
As said by others, full suspension bikes are generally unpleasant to ride on pavement for any distance. When the suspension is working hard over undulating ground you barely notice and it is an advantage, but when you can discern movement while riding on the road, it is frustrating and feels like you are being slowed down. A 29er with 29X2.3 or whatever would ride like a Lay-Z-Boy, but still be efficient on the road. A good quality 29er would also likely come with a decent suspension fork, which is not generally true of hybrids (poor suspension forks) or gravel bikes (usually rigid forks).