I'm doing something similar with Vue. The modern mode flag makes it easy. It creates two builds. Using the module and nomodule fags IE users get transpiled code and everyone else gets just minified code.
I've found that its not foolproof though. It can generate some wierd errors when it converts a generator into a jumbled mess of semi-equivalent functions. Luckily I don't have to worry about it. I got the go-ahead to drop IE. Giving them the transpiled code is literally all we are doing for them. If it breaks, its not an issue.
IE users see a big warning on a login screen telling them in no uncertain terms that their browser will not work. Its kind of disheartening how many lines I still see in the server logs with "Trident". I don't know why these people are so stubborn and insist on using a slow browser on a website that tells them that they will experience errors on that site.
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