So, the web uses a system called chain of trust. There are public keys stored in your system or browser that are used to validate the public keys given to you by various web sites.
Both letsencrypt and traditional SSL providers work because they have keys on your system in the appropriate place so as to deem them trustworthy.
All that to say, you’re always trusting a certificate authority on some level unless you’re doing self signed certificates… And then nobody trusts you.
The main advantage to a paid cert authority is a bit more flexibility and a fancier certificate for your website that also perhaps includes the business name.
Realistically… There’s not much of a benefit for the average website or even small business.
Plex is moving in the app direction… So Plex is probably moving away from what you want despite being one of the easiest options.
It would probably be helpful to know what you’re trying to accomplish beyond “what”. Like, why do you want to host your music and play it via a web browser.