Last year I built out a custom rich text editor using Slate. It certainly is powerful and fit right into the existing React app. We did, however, run into a handful of issues. The biggest thing with Slate was the need to implement most things myself. Slate functions great as a core for editor operations, but beyond that you're pretty much on your own. Some interactions were easy but more complex wysiwyg editing became difficult to reconcile with Slate's API. I've even needed to patch Slate once or twice to get things working properly.
I don't have any experience with TinyMCE, but I would imagine there are similar issues to do with rich editing experiences. My best advice is to pick a simpler solution if you can. If something like QuillJS satisfies your needs with its out-the-box experience, then that may ne netter than investing time building your own edjtor with Slate. If, however, your editing is unique and/or complex, then Slate provides a decent base to work off of.