This is the patch that has invaded.

Hello

Wow that is a lot of mint!  Too bad you can’t harvest and sell it. As you can tell mint is very invasive, and unfortunately difficult to eradicate.
I believe you only have three routes here: you can hand dig it out, which would be very laborious and then still you would not be able to get all of the root pieced as they can go 3 feet deep.

You can spray it with an herbicide, but you would have to be very careful around the tree and other plantings.

The final route would be to cover it all with old sheets or similar and then cover the sheets with a thick layer of mulch, basically you would be smothering the mint, but you would want to leave it undisturbed until next spring as it takes a while.

If you would like to have some mint for the future, dig some up and put it in a tall pot and make it behave.

All the best

Kathleen