I just use plain sucralose (actually Sweetzfree) to sweeten my cranberry sauce. It is slightly less firm than cranberry sauce made with sugar, but not so much that I've ever felt the need to add anything to thicken it. Some people add sf Jell-O to it, which in your case, might help overcome the bitterness, as well as thicken it a bit. I think if you googled something like "sugar-free cranberry sauce recipes" you get a lot of results.