Hello Darkness,
how did you proceed after installing Asterisk? I’m doing exactly the same as you. I already have a working Asterisk 20 on Raspberry Pi 3.4 as an image. The key point is FreePBX 17 from the GitHub repositories, see also "Error 'Ramsey\Uuid\Exception\UnsatisfiedDependencyException' occurs during FreePBX installation after entering database details" - #3 by Niclas
Perhaps you know this error and have already been able to solve it.
I also plan to provide an installation script for Asterisk and FreePBX on Raspberry Pi.