You have tried a lot of things there.
Stiff chain link? Remove the chain and try each one for freedom. Check the joiner link when you put it back on.

Is the chain the right width? A 9 speed chain on a 10 speed cassette frinstance will give you grief - if it were to work at all.

Chain length - I use chain checkers - don't mention them in the mechanics forum whatever you do! Measure with a ruler to be absolutely sure. For reasons which I finally understand but take too long to explain - the Shimano style checker is the only reliable one, but a ruler is failsafe.

The remaining common factor seems derailleur hanger alignment. The only way to do this well is with an alignment guage. Since I bought mine pretty well every bike I get is out of line until I tweak it. Sometimes it makes a difference, not always, but sometimes. Don't bother checking by eye - you will only see what you think that you should be seeing.

Test your shifter by pulling on the cable to simulate the return spring load - shift up and down. Try with a light pull and see if the shifter internals are working properly - sometimes the work ok but only with a heavier pull than the derailleur is providing.

Another thing to try is - can you shift gears up and down by pulling the cable yourself (not with the shifter). If you can then this points to the shifter being erratic.