Thanks for the feedback, Tom. I'm an amateur too.
I am cooking St. Louis cut pork spare ribs, not back ribs. I've tried trimming them myself from a full rack and buying them already trimmed. I see no difference in the result.
Your description of how you cook yours is almost exactly the same as the approach I have taken, with similar variations along the way. My ribs tend to fall off the bone, but they fall off in pretty dry chunks when I bite into them, not tender, juicy pieces of meat. As I mentioned, I check for doneness using the standard tests at 4, 5, and 6 hours. I never get the expected result with the toothpick test or the bend test, but yet when I pull them at 6 hours, they're dry and overdone.
By contrast, when I have spare ribs in a decent Texas BBQ joint (there are lots of them here in the metroplex), they are juicy, tender, and flavorful. Sometimes they fall off the bone, sometimes they don't, but they're always moist and tender. I never get moist, tender ribs when I cook them on the SI. I do know that the BBQ joints here in Texas spritz, wrap, and sauce their ribs while they cook them. I'm pretty sure that makes it very hard to end up with dry ribs. Everything I've read here suggests that with the SI, you don't need to do all that because of how tight the unit is and how well it preserves moisture. I guess I'm just not seeing that.
And please don't get me wrong: I love my #2 analog. I get excellent results on almost every type of meat I cook -- just not spare ribs. I don't like back ribs because the butchers here leave too much meat on them to increase the price, and I can't cook them like ribs. I always end up with what looks like a small pork chop on one end of a rack of them.
Thanks again for the feedback. Next time I cook a rack, I'll post back and share my results.