If you think keyloggers require software running on your physical keyboards you're in for a rude awakening.
Keyloggers are almost always at a pure software level and are conceptually simple to make. So simple that in fact, it's the same thing as running a regular application with background shortcuts. The only thing that is different is that regular apps aren't saving/recording anything, they're just listening for you to press cmd+whatever.
It takes maybe ~10-15 minutes to make a keylogger in Python that could run on any computer, mac, windows, or Linux. Maybe a little longer if you wanted to use a compiled language and properly hide it.
Sorry to burst your bubble.
- A software developer
You can have both though. Just add some random menu in the settings that turns bright red when using a non-certified component so security can be easily verified, but don't needlessly lock people out and charge $500 to fix a $10-50 module on a $1000 phone
Edit: Adding on to this, Ifixit isn't outlawing verification, the above example of whatever red warning is a clear way they could keep it.