I recommend Redaxscript
I found it years ago, when i was looking for a cms that is mostly written without any frameworks, easy to create modules and themes for it. I always got lost in wordpress, this one checked all the boxes for me.
To be fair, i loved it so much i contributed to it on github. I'm still following the development (refactoring happening) and gonna definietly contribute to it (when i have the time). (I found it really easy to read the source code and modify since lack of extra frameworks)