Define long distances? 50+ miles? 100+? Multi day grinders?
What type of terrain/elevation changes are you riding?
I went through similar issues with my bikes.
1. I am getting arthritis in my hands and wrists, so washboard and washouts can be very painful on faster descents.
2. After my gravel bike was beating the heck out of me on 85 to 100 mile gravel grinders with 30+K climbing, I installed shock absorbing padding
on my bars then wrapped with bar tape. This helped a lot for general vibrations. Then I installed a PNW Coast "suspension" dropper post. This helped a bit more.
3. I still hated washboard and washouts on descents because I couldn't ride over them at speed and had to slow down too much. I thought about adding short travel suspension on the front, but changed my mind and went the 29er hardtail route based on the bike I posted pics of.
4. On the 29er, I put purchased both a carbon rigid fork, a Rockshox SID Ultimate 100mm travel fork and another PNW Coastal suspension seat post. With the rigid fork, I quickly discovered that having 50mm to 2.2 low rolling resistance tires on this bike made a lot of difference in comfort and the bike's ability to handle washboard and small washouts. This bike descends so much faster than my gravel bike, yet still climbs quickly.
5. For the 29er, with the SID on front, I found that I could start riding the bike on trails with some rock fields, roots and smaller drops. The suspension seat post sucks up a lot of the jack-hammering on the back end when you are sitting down, but I find that I am off the seat when I am riding over roots and rocks, so most of the pounding is transferred to the bars. The SID fork gobbles up almost all the roughness of washboard and washouts. With the bike set up with "H" bars it still handles well and the bike rides well for 100 mile rides. - The bike, set up with the SID and tires that handles more rocky trails adds several pounds, so it rolls and climbs slower.
6. I ride a hardtail these days for probably 70% of my off-road trail/gravel road riding. The rest is on my XC FS bike. Full suspension bikes add a lot more weight, climb slower, and to me, really aren't any more comfortable on washboard and washouts. They do shine on single-tracks with diverse terrain, but I usually only ride 30+- miles on FS bikes and I expect to get beat up and tired anyways.
