In general you want to put things like item statistics, level up experience point thresholds, and any other “pure data” into some kind of text file or database, and then read it into the memory on load or when you need it.

Look into exporting and importing JSON objects, and write ‘types’ in your code that are read in from JSON files. Then when the game or level loads, you read in all of the item statistics, for example.

Nice job! The presentation is very readable despite the inherently cluttered playspace of a TD game