I had the same trouble with a standard Swift. I built a robust training bike and a Brooks seemed an obvious choice. After a wet 100km it was sagging badly.
It seems they mustn’t get wet. So if you are on a multi day ride and it gets soaked on day one, you’ll have to abandon or carry a spare saddle.
But actually what you do is spend £200 on the saddle and then another 5p on a Tesco carrier bag.
They are high maintenance. I find it’s okay if I have guards, the thing is treated and a small saddle pack (heavens forbid) helps keep that rare old British thing at bay.
Not sure if it’s rose tinted specs or what, but my old ones were as tough as old boots.
Anyway I let mine dry, with a bit of paper stuck up there. Then tensioned it and added more Proofhide. Mine still looks saggy, but it is comfy and still resides on my winter bike
Edit; I think all we’ve done is speed up the breaking in period, though mine was a delight out of the box. And though I feel your anguish I don’t think a mountain bike is a place for a Brooks.