If you create a standalone app and leverage the Discourse API only, you could theoretically build a complete new app in React. That would be a crazy amount of work, but believe there have been some attempts to do this in at least react native in the past? The challenge is the massive amounts of ongoing maintenance to keep feature parity. That example hasn’t had an update in a while? See Lexicon: a customizable native mobile app for your Discourse site - #22 by Kuro22
Do not attempt to mix the frameworks. I have tried using a bit of Bootstrap with Discourse but results are very unreliable. You might get some things working. I don’t think mixing React will work at all?
My advice: learn Ember and the official plugin eco-system thoroughly instead.