I also tried using a soft proof profile from Saal Digital some time ago and it looked very wrong to me. I can’t back this up with any references at the moment (so please correct me, if I’m wrong) but my understanding back then was, that their profiles are specifically made for Adobe applications and do not work correctly with darktable (maybe due to these missing options that you mentioned).
Even without using the soft proof, I got prints that looked relatively close to what I see on my screen, though. You need to make sure that the “auto-enhance” option is disabled though, could it be that this was on in your case? At least with a different printing service this “feature” once completely messed up a few prints of mine.