How to break out of a for loop on button press?

[woooee]

If you want to use after() in the way the OP was going, it has to call itself.

def process():
    ## replace for with after
    ##for i in range(100):
    if some_condition:        ...
        root.after(milliseconds, process)
        # playing a midi-file here 

Or look at one of the tkinter video players like https://pypi.org/project/tkvideoplayer/ I don't use them so don't know, but this one says it has pause and stop commands.