Migrating from commercial music notation software to free alternative

[ah]

A friend of mine dislikes composing music on the commercial software he 
owns (Si..us). The main reason is that he is into modern, avant-garde 
music and the notation on his current system is just not there yet or 
what's offered does not satisfy him. He is a professional music composer.

I have no idea about music composition.

He asked me for helping him and I see I have two choices, either I 
experiment writing some plugin for his current commercial software I 
mentioned, which is uncharted waters for me plus only available on an OS 
I don't have, or ...

... using a TeX-based or TeX-like (free) music composition system, if 
available and doing all programming, templating etc. in this myself. 
Advantages: platform independent and free.

I can manage any installation challenge and possibly I can persuade him 
to free himself from the GUI for the sake of programmability and 
typesetting/engraving quality.

So, my question is:

Has anyone here had any experience with both the free and the commercial 
music-composition softwares and can give some advice whether it's wise 
to dump the commercial for the free without compromising typesetting 
quality which is my primary metric.

And also whether there will be lots and lots of symbol libraries for him 
to choose from?

(also posted on 
https://tex.stackexchange.com/questions/495468/migrating-from-commercial-music-notation-software-to-free-alternative)