Cook a rack of baby backs at 275° and don't open the grill until it hits the 3 hour mark. Test for your desired tenderness and cook no more than 45 minutes more. Report back on your results. That should solve your tenderness issue.
I did two racks like this a couple of days ago. I did three hours and then added 15 minutes to set the sauce. They were good, but the bark was really tough. That may have been due to the rub I used. It was the first time I've used it and it may make more bark than we care for. I monitored the pit temperature carefully with a FireBoard. Its boiling water test checked out fine.
I'm going to do another run like this with a different rub.