I bought a C17, did not like it, the rubber was much stiffer than other C17 saddles that I had looked at, it was to hard.
Later, I bought another C17 all weather version. Did not like it either, even though the rubber was softer than the first. Just mentioning this because the All Weather version has a smoother surface than the first one I got that had the regular surface. You might consider looking at the All Weather version to see if you might like it. A non-leather saddle might hold up better for that much riding for your planned trip, it would be much more maintenance free than the leather saddle.
I agree with Dan, the saddle clamp and the saddle rails should mate nicely without causing undue stress.
I have never had a saddle rail break. I would think that a Brooks on a suspension seatpost would be less likely to break because the suspension should reduce some of the stresses that the saddle has to deal with on a rough road.
I know you said you already ordered a saddle, but since you asked, I have not used a Cane Creek short travel seatpost, so I can't comment on how that compares to a sprung saddle.
But I have used a Flyer and several Conquests. Almost all of my riding is on Conquests. I think the Conquest and Flyer share the same springs. I am about 80 to 85 kg. When I put my weight on my Conquest and sit somewhat upright, I think that I deflect the springs by 5mm at most. My point is that these saddles do not deflect that much and I suspect that the suspension seatpost will give you much more cushioning.
Where I think that the springs perform best is on paved roads with a rough surface. In USA we have some roads with a surface called chip seal, those roads cause a lot of vibration and a sprung saddle is pretty nice on that.
I do not know what your planned tire width is, but the wider the tires, the more cushioning you get from the tires.
I am considering buying a suspension seatpost for my Lynskey touring bike, that has 37mm tires and I usually pump them up pretty hard when I am carrying a load.
I have ridden my Nomad Mk II with heavy loads on 57mm wide tires and the only times that I thought a suspension seatpost would be nice on that bike was when I was on the type of roads that cars can't drive on because the road was so rough.
First photo attached, this was the only time that I thought a suspension seatpost would be nice on my 57mm tires, look at the road surface in the photo. When you see something in the road that is smaller than a tennis ball so you do not bother trying to ride around it, that tells you that you are on a a rough road.
Second photo, you can see how rough that chip seal road is, the shoulder is much rougher than the road surface where traffic has smoothed it out more. Here is where I think a sprung saddle works best to smooth out the vibration.