Thanks for your idea @rdswift !
It seems to work if you do not break up the lines (for better readability).
This is the 1-liner that we can copy & paste 1:1 for all current Release Types:
$setmulti(releasetype,$map(%releasetype%,$if($in(%_loop_value%,album),Album,$if($in(%_loop_value%,single),Single,$if($in(%_loop_value%,ep),EP,$if($in(%_loop_value%,broadast),Broadcast,$if($in(%_loop_value%,compilation),Compilation,$if($in(%_loop_value%,soundtrack),Soundtrack,$if($in(%_loop_value%,spokenword),Spokenword,$if($in(%_loop_value%,interview),Interview,$if($in(%_loop_value%,audiobook),Audiobook,$if($in(%_loop_value%,audio drama),Audio drama,$if($in(%_loop_value%,live),Live,$if($in(%_loop_value%,remix),Remix,$if($in(%_loop_value%,dj-mix),DJ-mix,$if($in(%_loop_value%,mixtape/street),Mixtape/Street,$if($in(%_loop_value%,demo),Demo,$if($in(%_loop_value%,field recording),Field recording,))))))))))))))))))