Im on the other side of the fence that pork belly is on,,,
He is very right about everyone having there own idea of what they like,,,
Me i would tell you to drop the temp to 220 and mine go anywhere from 6 1/2 hrs to 7hrs and usually break in half when i pull them off the smoker,, but i love them that tender,,, i think you toughness means you need more time in the smoker,, other than the water in from the start everything sounds right to me..
but hey thats what makes this fun for me, trial and error..
Try this do two sets of ribs, cook one for what time you think they should come out and leave the others in for another hr and them compare the two and see if there is a difference you like...
let us know ..
Good luck keep at it you will figure it out
PJ
OH, P.S.
My family ate a bunch of tough ribs before I figured it out, you are not alone. now they love them,,,