"What advice would you give somebody coming from Facebook, Instagram, Xitter, Reddit, and/or TikTok who's starting to check out fedi?"
Racism, ableism, ageism, queer and trans phobia, misogyny and other such bigotries you hold will be challenged, unlike commercial social media which lives on these bigotries.
Try to challenge yourself before joining, or learn how to take such things with grace and acceptance, otherwise you'll find yourself blocked, or with the fascists or other bad folks pretty quickly.
You're joining a community, unlike commercial social media what you say, think and do will be met with the appropriate actions. Therefore which community you stay with is up to you and how you behave.