Useful or not, from you.
mkcert mkcert -install keeps telling me that "keytool is not available", when it is?

I keep getting:

Warning: "keytool" is not available, so the CA can't be automatically installed in Java's trust store! ⚠️

When it plainly is available, I can just type "keytool" and it works. JAVA_HOME is set just fine. I'm really at a loss because everything else can access it just fine.

That's a useful answer
Without any help

Weird! Can you give us the output of which keytool and echo $PATH? Are you running mkcert with sudo, maybe?