The best alternatives to BriefKey are iTunes, Slash Keyboard, and Typewise. If these 3 options don't work for you, we've listed a few more alternatives below.
What do you think of BriefKey?
Hex— Smarter analysis, faster data science with a little AI Magic
Boosting productivity 2-3x with text prediction software. Available as enterprise solution and API for developers. Previously built keyboard app for iOS and Android users.