well, here's the kicker. you can only repair what you have. so even if the ssd is removable, you can place an order only for the same capacity one. unless you have access to someone else's serial number who happens to have a 2TB mini.
this is truly madness. apple still has its profits through this program, then why the hell they don't allow the poor man to choose from the compatible SSDs?
I just tried and it allowed me to add a 2TB SSD to my cart even though my M4 mini has a 1TB SSD.