There's no adjustment on the steering gear. Look up the symptoms of a failing rack and pinion, see if any of those occur.
Low fluid should first manifest itself with a failing pump (grinding/whining noise).
Have qualified mechanics done all the work you listed? It's hard to believe a good mechanic can't diagnose loose steering. Any tire/alignment shop should be able to figure this one out with a test drive and a few simple tools.
Most of what would cause loose steering that you haven't replaced already would show other major symptoms and should be easy to find, like loose control arm bushings and worn ball joints. And I wouldn't discount early failure of a new tie rod.
If I were looking at it at home, I'd get an assistant to turn the wheel (engine off, car secure) while I got on the ground with a light to see what's moving and what's not. If the steering shaft input to the rack and pinion is rotating but the tie rods aren't moving, it's your steering gear. If the tie rods are moving but the wheels aren't it's a suspension component. A pair of large channellocks and a crow bar can be used to check tightness of joints and bushings, but leave that to the pros.