I am going to share my opinion. Everything I produce on my own (including my Mastodon messages) is dedicated to the public via CC0.
For software I am a copyleftist, because code can be obfuscated and I believe we need protections to ensure software remain accessible to future users. But copyleft is a form of copyright, and that means it is backed by a state willing to enact violence to enforce it (to clarify, I consider being sued or sent to jail to be an act of violence).
Aside from CC0, I can personally contribute to content projects that are licensed under CC-BY and CC-BY-SA. The reason is not because these are common sense decisions to share knowledge (though of course it is), nor is it a dedication to free culture (applicable as well). It is because I can’t be bothered to enforce copyright, and it would be a lie if I threatened people with legal action; I am never going to use a government institution to hurt a person because they decided to “use” something I created.
So there is a practicality there. Even if you don’t share my feelings about state-sponsored violence, consider the alternative of full copyright: does your instance collectively enforce it for individuals? Is it up to each individual to enforce their own (I am aware of social.coop, so that seems odd, ne?)? Is anyone going to fight a fair use battle with a corporation that has actual lawyers on staff?
I know this is hard stuff for folks to consider. Once I decided to not deal with copyright anymore, my life got a lot easier.
I would go with CC0 or CC-BY; that is my professional recommendation as a webcrafter and free culturalist.