Gears shift perfectly when suspended on rack, but not on the ground?

While I'm not sure this qualifies as an answer, because with such questions there is usually a lot of guessing, your issue sounds like a flexing issue or a worn rear-hub.

The flexing could happen because of your weight (maybe fatigued frame? or just flexible) resulting in slightly changed geometry while riding vs suspended mount.

A worn rear hub could cause a wobbling or light tilting in the cassette while under load.

I would also check the cables while the bike is mounted on the stand and see that the clamps or the bike position (handlebars sitting at a hard angle, pulling on cables) are not somehow affecting the tension in the derailleur cable.

Running out of stock of useful crystal balls it is hard to guess what the true cause is, here. But here are some things to check.

It partly depends on whether you use a "low normal" (without cable tension the cage is pulled to the sprockets with more teeth) or "high normal" (the spring pulls the cage to the sprockets with less teeth) rear derailer.

If it is the latter, you can try disconnecting the cable to see whether it shifts to end. If not, re-adjust the limit screw. If you then need to loosen the screw so far that the chain is frequently dropped off the end of the cassette, most likely your derailer hanger is bent. In this case the pulleys are not parallel to the cogs, throwing the chain off the cassette. If however it works properly disconnected, the might be friction in the cable housing that pulls the derailer inwards; read on.

In case you use a low normal derailer, don't disconnect the cable. You may want to check the same points as mentioned above, but before you have to make sure the cable tension is right. There might be just enough friction in the cable housing that some flex in the frame accounts for insufficient tension, especially if the shifter cable is routed around the bottom bracket. Try increasing it a bit. You may increase it until the other gears don't shift properly anymore, then back down slightly.

Note that if increasing the tension indeed fixes shifting to the highest gear, this will probably not solve all your shifting issues. Your trade off will likely be non smooth shifting to the lowest cog. Replace the cable housing (and the cable) to fix.

When you say 'on the ground' I assume you mean you are riding the bike and testing the shifting, and when the bike is off the ground you are spinning a crank with your hand.

The major difference between riding and spinning the crank manually is the forces the drive train has to transmit. I.e. shifting is effected when there is significant tension on the chain.

I suspect worn chain and cassette. If the bike has significant mileage check the chain stretch for elongation and the sprockets for signs of wear. I'd also check the state of the derailleur jockey wheels.

I'd start with the derailleur hanger. Usually fixes my problems when everything seems okay, but the bike still doesn't shift well.

I'm having a similar issue, is your bike noisy while shifting? While I've finally gotten my rear derailleur to shift through all gears without slipping, it's pretty loud when it does shift. This is after cleaning, and lubing the chain and getting all the cogs lubed, wiping the excess off. Fun fact though, to get the chain on the outer sprockets I had to start by loose both the high and low limit screw and fiddle with the barrel ferrule that the cable runs through. Be bigger cog was reached by playing with the tension on the high-limit screw and the ferrule, once I got that done, I shifted down to the smallest sprocket and fiddled with the low-limit screw. After a little bit I was able to shift almost seamlessly, albeit a little noisy, from each gear. I hope this helps.