Ok, I think this has some way to go still. Compared to MessagEase quite a bit of features are missing (the combine button which gives instant access to a whole lot of special characters, many settings, also do I have to press # to access numbers, etc). That said, I’ll keep this installed and hopefully contribute to its development if I can.
Actually I just found out about thumb-key that seems to fit the bill perfectly. In theory at least.
Borderlands Borderlands 2
Then if you’re still interested, Borderlands Pre-sequel
And if you find yourself really into the series, Borderlands 3