Welcome to TFP, I'm not a builder or repairer, just DIY guy. Do you know if your decking was a part of the pool (structural) if so then I haven't a clue. If not and since your not losing water then it may be possible to simply cut a wedge where the crack is, repack the base underneath and refill with concrete. The sides of the old concrete where they meet the patch would need to be cleaned and have a bonding agent applied to help with adhesion at the joints.
My greatest concern would be why is it sinking/lifting and trying to fix that when you patch it.
The nice part about where your crack is- next to a joint already! Possibly remove all concrete from that joint over to a straight line 6-8 inches beyond the current crack edge. It would like they added an extra expansion joint.
Depending on concrete thickness I would also drill laterally into the old concrete and pin it so it doesn't shift.