CPX reference was to weelittledrummer.

As to that intonation issue -

It sounds as if the luthiers you have used so far are agreed that the issue is saddle positioning.

Next step therefore has to be either removing and repositioning the bridgeplate, or routing a wider saddle slot and then partly filling the new widened slot so as effectively to reposition the saddle. This in turn may mean (depends on spacing) plugging and redrilling (and tapering) new pin holes.

You can get a fair idea of how far out the guitar is by comparing the harmonic at the octave with the fretted note at the octave. You can retune the adjacent string for a reference if you don't trust your ear or an electronic tuner. If the difference appears large you may even be able to measure the length difference required with a tape measure. Always do this with new strings on - old strings may have become non-uniform in diameter. If the octave is right, you have a fretting problem.

You can also determine the right position experimentally. You will need an old saddle. File it down until it roughly does nothing - simply fills the saddle slot up to the level of the wood. Now take something like a giant paperclip bent straight, or a long thin nail with the head cut off, or a plastic cocktail stick. Put it pretty much where the saddle slot is. Now slide it (carefully, so as not to break it if it's plastic) backards and forwards until the harmonic octave is the same as the fretted octave. That is where the saddle (for that string) needs to be.