Appdata will stay there as it is directly linked to the OS. You can theoretically change it however it is not advisable without experience and can often lead to either lost data, or system instability. Otherwise you will still need to manually relocate all of the already installed third party programs even after changing the default settings.
64GB should ultimately be enough for just the OS and basic AppData assuming you create all of the installations elsewhere.
Uninstall and move the install location of programs already on your C drive. This may require you uninstalling and reinstalling.
Regularly perform Disk Cleanup and SSD TRIM (TRIM is usually automatic in the background).
Remove redundant application data and remove unnecessary programs.