People have been on much higher doses for much longer for treatment of certain types of cancers etc...
I agree, on 40mg of pred, I went from 15 watery BM's a day, to one solid
I have been tapering for the past month and am now down to 20, but have the same concerns as you
You definitely do not want prednisone to be your long term treatment option, so I would agree, wait until the Humira kicks in and very slowly taper, if you notice a flare as you taper, you may want to talk to your GI about adding other meds to the humira (my doctor put me on 6mp and humira)
I wish you the best