8 hours? Ouch. I've done a couple of rides like that and there's no amount of suspension that would help. 3 hours or 30 miles is my go to ride. I can do more but it becomes less fun/mile.
Yes, I'm married to an insane person. That's for sure. But still, my point is that you could upgrade to an inexpensive used full suspension and probably find a significant increase in comfort and ease.
Basically, you don't have to spend big money for a big upgrade if you're going from hardtail to full suspension.
That was actually the deal he and I made. He really wanted to upgrade to a particular expensive full suspension bike, but I talked him into trying an inexpensive full suspension first. No point in investing in this crazy ass bike if the full suspension isn't as amazing as expected. He could get a nice brand new hardtail for so much less.
So the $800, 20 year old used bike was just supposed to be a trial bike, but he absolutely fell in love with it. It was such an upgrade in riding experience, especially given his finicky knee, that he decided that that much of an upgrade was enough for him and he couldn't imagine that spending several times more would be that much better.
So we invested in some upgraded components and he's happy as can be. I just again asked him this year if he would want to upgrade to the new full suspension and he was like "shh...don't say things like that in front of her, just because she's old doesn't mean she isn't beautiful."
He seriously loves that bike.