Some guitarists have experienced this. The string action is catastrophic, in some places much too high, in others so low that buzzing is inevitable. All the advice you've received in forums and specialist journals has been of no avail. Turning the truss rod either one way or the other, sanding down the bridge inlay only helped for a short time and the piece of veneer under the bridge was no remedy either. Now the only solution is to visit a luthier.
Neck reset is about correcting the neck angle of the guitar. A neck reset is a serious surgical intervention in the guitar. Applied to humans, it could perhaps be compared to the insertion of an artificial hip joint. The fingerboard is detached from the top and the neck from the body. Then corrections are made to the dovetail joint and the neck is glued back into the body at the correct angle. In most cases, the procedure is combined with new frets, as the fretboard has to be trued. In rare cases, a new fingerboard may even be necessary.
How does it happen that a guitar changes in such a way that the neck angle is no longer correct or no longer fits the instrument and the string action becomes so high that playing is only possible with increased effort?
First of all, there is the string tension, a force that constantly affects the top. Over the years, many guitars develop a bulge in the soundboard at the height of the bridge due to this string tension. This bulge, also called a "belly", causes the bridge to no longer sit correctly. Since the string action is determined by the height of the two support points, nut and bridge, the result is a higher string action. At first you can still sand down the bridge inlay, but at some point you can't go any lower.
The next victim of the string tension is the neck. Some old guitars don't have an adjustment rod, so readjustment is not possible. If the neck wood is relatively soft, the neck can give in to the string tension over time and bend completely. The string action is then only acceptable in the very lowest registers and increases radically towards the transition. If a truss rod is present, it sometimes only works as far as the transition to the body, in which case the neck tilts towards the strings and a kink is created at the neck-body transition.
It is possible that in this case the neck is straight up to the 12th or 14th fret, and then begins a ramp upwards from the bend towards the sound hole. The string action then becomes continuously higher from the first fret towards the transition. There the strings lie too close to the frets and cause corresponding noise. If you increase the string action at the bridge, this clattering disappears. At the opposite point of contact, the nut, it still works in the first position, but in the middle positions, the string action is much too high and the guitar becomes very difficult to play. In addition, intonation also suffers from too high a string action, but that's another chapter.
The third evil that requires a neck reset is that the tenon of the dovetail joint has shrunk over the years or was slightly undersized in the first place. If the tenon was too small, the voids have been filled with glue. This deteriorates over time and can no longer hold the joint. Then the neck loosens and also tilts forward.
The removal of the neck is a difficult matter. First, the neck including the fingerboard is detached from the body with the help of steam. This process often takes several hours, during which the joint is loosened bit by bit with the help of wedges so that the steam can penetrate deep enough. It is helpful to use wedges, which are carefully pushed further and further between the top and the fingerboard until the neck and the glued fingerboard come loose. Then the glue residues are carefully removed. Now comes the part that really requires craftsmanship from the guitar maker. The neck must be refitted at an angle that is favourable for the desired string action. Some wood must be glued onto the cheeks of the tenon, usually a veneer will do. Then material must be carefully removed from the front of the tenon and the foot of the neck in such a way that the neck is later tilted further back. Finally, the veneer glued to the cheeks must be removed in the opposite direction. Only then can the neck be glued back in place. Often the fingerboard is no longer completely straight. In this case, the frets are removed and the fingerboard is trued. This is usually done with a plane, but since inlays are often already present in the finished guitar, sanding is used in the case of repair. To do this, a sturdy piece of wood, about the size of a fingerboard but much thicker, is covered with sandpaper on the underside and this tool is passed over the fingerboard again and again with a sure instinct, checking the angle by visual inspection in between. Finally, the fingerboard is re-fretted.