Welcome to the world of Mitsu transmissions...!
Usually it's the transmission fluid that needs a change. But you already changed that multiple times.
Could be a worn out syncro, too. Then a rebuild is necessary.
The linkage can be adjusted. Did it in the past, forgot how to do it, now need to do it again. Darn brain's a sieve.
Also the shifter assembly can wear out and needs a rebuild. Just enter "dsm shifter bushings" on youtube and you'll get myriads of how-tos on that. Our CSM shifters are close enough to DSM shifters. A specific video can be found here: https://www.youtube.com/watch?v=RDa3tW3m3-c