A ticket can be answered on time and still leave the customer unsure, annoyed, or forced to chase again. That does not mean SLA data is useless. It means it needs company.
Measure what the customer actually experiences
Useful experience measures might include confidence in the update, ease of getting help, repeated contact, waiting without clarity, or whether the customer felt the team understood the issue.
Keep it simple enough to use
XLA work should not become another performance theatre. The point is to find the experience gaps that matter, then use them to improve service decisions.
Use the data to change behaviour
The best measure is one that helps a team act. If nobody knows what to do differently after seeing the number, the measure is probably too far away from the work.
Useful question: What does your current reporting say is fine, while customers are telling you it is not?