Baby backs are in my experience trickier than spareribs. They're twice the price, aimed at a market that can't be bothered with spareribs. With the exact trouble you went through, you could have cooked spareribs. They can be cooked similarly, with baby backs needing less time and less rendering.
On the other hand with "grade inflation" what used to be sold as St Louis spareribs is often sold now as babybacks. A pig rib is a pig rib, I like 2 1/2 to 3 lbs trimmed, best.
I personally brine many meats, but never ribs. On the other hand, the salt is unpredictable by eye, going for a dry rub instead. So I measure 0.8% sea salt by weight (one would use more for boneless meats, but this accounts for the bones) and apply before applying a further salt-free rub of chiles and/or black pepper.
At 250 F I'd agree you went for a long time. On the other hand, ribs look done when they're done. Peek earlier?
A classic method for ceramic cookers and spareribs has been 3:2:1 where the middle 2 hours is in foil, perhaps sauced. With indifferent meat this is probably a great idea, but with the most flavorful pork we find this comes out mushy and hides the pork itself.