I would want to tell my past self that I’m trans and they should take HRT ASAP and that all my friends and family are accepting and that I don’t need to wait until I’m 30 to come out.

Ask my future self. Telling my past self something is very likely to have no affect on my life (what if past actions only affect that “me” and not me-me), whereas asking my future self something will always end up with me gaining information - no guarantee its relevant, but it’s more likely to be.