I came here to request the very same before finding this post.

My original suggestion on the iOS Store app review:

Please add support for a native iOS keyboard
Man where to start... there are sooooo many things I love about Textastic I don't know where to begin...
Just try it, you won't regret it 👍🏻
@Developer(s) please make my dream come true to have the ability to use your wonderful keyboard extension outside of app as a native iOS keyboard as I'm completely in love with its functionality as a power user.