The main benefits to paying for certs are

• as many said, getting more than 90 days validity for certs that are harder to rotate, or the automation hasn’t been done.
• higher rate limits for issuing and renewing certs, you can ask letsencrypt to up limits, but you can still hit them.
• you can get certs for things other than web sites, ie code signing.

The only thing that matters to most people is that they don’t get cert errors going to/using a web site, or installing software. Any CA that is in the browsers, OS and various language trust stores is the same to that effect.

The rules for inclusion in the browsers trust stores are strict (many of the Linux distros and language trust stores just use the Mozilla cert set), which is where the trust comes from.

Which CA provider you choose doesn’t change your potential attack surface. The question on attack surface seems like it might come from lacking understanding of how certs and signing work.

A cert has 2 parts public cert and private key, CAs sign your sites public cert with their private key, they never have or need your private key. Public certs can be used to verify something was signed by the private key. Public certs can be used to encrypt data such that only the private key can decrypt it.

You can, and for Linux generally have to, manage your own secure boot keys and signing your own kernal, united, modules, etc. Conacal and Red Hat have signing keys iirc, but distributions can and do get the shim boot loader signed so secure boot works. The arch wiki has a page on how to setup secure boot . Many distros installers do end up signed as well so you can go through the full install process with secure boot enabled.