They may be acting facetious, but "guys" is a gendered term and some folks don't appreciate being referred to as a guy. Just like some people don't like being called "dude". But that's person to person, anyway. To answer your questions from my perspective (I'm a nonbinary millenial fyi)

Your pronouns are whatever feel reflective of your gender. If you feel that you're a cisgendered man (someone who aligns with their gender assigned at birth), then yeah he/him is probably accurate. It's whatever feels most authentic to you.

As for telling people your pronouns, I think it honestly would be really nice for more cisgender dudes to normalize that. It's pretty uncommon for men to do in my experience, and I think it would show that you want to be respectful of gender nonconforming folks. Also if you share first, it may actually help people to feel more comfortable around you, and then they'll likely share theirs so you won't have to ask.

As for pronouns irl I've only really met folks who go by he/him, they/them, or she/her. I am not really deep in the queer community, though, and I think it's more common to find folks who have other different pronouns in those spaces because they feel comfortable using them. It probably also depends on where you live. I live in a pretty queer-friendly town and so trans and other gender nonconforming folks make up a decent chunk of the population.

In conclusion though, I think doing all of the things you just asked about is really great and more men should do. Be a model for other dudes; get people comfy with it because if anyone else gets weird/hostile about it you are in a place of privilege to push back on that, and more of that is needed, especially in the political climate we are living in rn