You don’t need to compile when changing JSON files.
For testing JSON, just change what you want to change and restart the game. The only reason you’d need to compile for JSON is if you’d never compiled the game in the first place.
The changes will be available immediately, though it depends on what you’re editing. IE: If you change an item’s weight or a creature’s description, for example, it will be immediately apparent. But if you change a location, it won’t change an already explored location, it will only change new ones.
I recommend creating a mod folder just for testing your changes, then making a world that uses that mod, so everything is fresh and organized and that way you’re not changing core files.
EDIT: Make sure what you’re editing is in the same directory as whatever one you’re playing, since you use the launcher but also have downloaded from Github.