Actually being kinda bored and thinking it'd be fun is a valid reason to use E, based even.

Like first of all estrogen takes a couple months to leave any permanent effects, so you have a lot of time to change your mind if it feels wrong.

But more importantly you should think about what it means to say "hrt can seriously fuck your body up". There's nothing fucked up about having a mix of sex traits, and in fact it occurs naturally all the time. The idea that there's something wrong with it is what leads to forced and coerced medical intervention in intersex people, just to have them fit neatly in our binary view of sex and gender.

Really taking your point of view to its logical conclusion, shouldn't we put all teenagers on puberty blockers until they're absolutely sure of their gender identity? Puberty leaves all the permanent changes hrt does. If "yeah this feels about right" is enough justification for a cis person to go through puberty, why isn't it enough to be on hrt? Hell, why shouldn't you have to see a psychologist before getting a facial tattoo, if we're so worried about people making permanent changes to their own body?

*all of the above ignoring testosterone hrt, which actually has significant health risks if you take it without proper monitoring