@kd@yiff.life honestly the few times i've come across this, i've ended up just sort of launching right into it. e.g.,

Good morning,

Thank you for taking the time to blahbityblahblah rest of letter goes here.

Best Regards,
Me

that sort of business i suppose? "to whom it may concern" definitely is like sometimes business speak for "i'm about to tear off your head and shit down your neck motherfucker" lol. a sort of cheerful "good morning" or similar is more casual but gets you out of this quandary.