I’ve done a few stage races, and split evenly between full sus and hardtail.
The key thing is that the bike is comfortable and fits well, the suspension does have it’s advantages in terms of improving comfort, but generally adds weight and most critically is another thing to go wrong. If the event has good neutral tech support then fine, but if it’s going to be patchy then a hardtail may be worthwhile. If you’re going for the adventure and not to win, then reliability is one of the key items to consider.
Are you planning on using bottles or a camelbak thing? Hardtails will usually have two bottle cages, whereas a full sus won’t and this may swing your decision. If you’ve already identified issues with your back, going back-pack free may be a good thing.
As you finish each day, make sure you get out of your shorts and clean yourself up as soon as possible. Try and use clean shorts each day, or at least wash and dry them overnight and use a cream with some anti-bacterial capability. Sudocrem is actually very good for multiple days in the saddle.
The suspension post isn’t as bad a suggestion as some people make out, but bear in mind if you’re trying to get back on on a hill that the saddle will be higher than normal and needs your body weight to bring it to the correct level, which can lead to amusing attempts to get going.