It’s not out of the question, but you might be surprised how limited the functionality of a keyboard extension can be. In the first couple years of their existence, to get one approved by Apple you had to actually include full keyboard functionality. I think they’ve eased off of that, but the extensions still have pretty limited ability to interact with the text input, and things like running actions would not be allowed.
I’ll take another review pass to see what, if anything, has changed about the APIs.
You might consider taking a look at the snippet library tip and see if that’s of help for any of your needs. You can put the drafts action (or shortcut) to copy snippet content in your Control Center for easy access. Might be of some help.