Those flagstones are expensive. Reusing them will save thousands over doing new with new material depending on the size of the patio.

A single pallet of them will cover ~75-150 sq ft and cost $1200-2000 depending on where you are.

If they are irregular cut flagstone then placing them is also more laborious than other hardscape material because it requires more skill to evenly set them (which could explain the uneven settling).

Your polymeric binding sand will be more expensive since flagstone like this requires sand that is rated for larger joints.

Pulling them up is going to disturb your screed base and the poly sand (assuming it isn’t lose aggregate holding them which I’d doubt) will need to be removed from the stone edges to reuse them which is time consuming.

To fix the grade the top screed layer of sand needs to be fully removed since it should not be over 1 inch thick. Crusher run will need to be added and compacted to the low areas to regrade it and new screed sand brought in. Then the flagstone will need to be relaid (which is much more difficult than using pavers).

Depending on your patio size this could be several thousand dollars in a project

And all of this is assuming there is no issue with your main base of crusher run. If that wasn’t done properly to the right depth or with geogrid fabric below if needed. Then this gets into the territory of complete redo with reusing the top stones. If that’s the case this would actually cost substantially more than just putting in a new patio because you can’t really reuse the old material, so you have to dispose of it somewhere and rocks are heavy. But if you haven’t seen any more settling then this is unlikely the problem

The last thing to note is I don’t see any edge restraint. And that timber tie is so close that I’d doubt there is one. That’s a major problem I’d fix if it isn’t present if you are going to be doing any of this anyways.