Try Boss Expertise. It adds a /expert command (goes both ways) & a button on Cheat Sheet if you use that.it doesn't work :/. Who have alternative tool/mod?
You may also want to keep an in eye on Expert Enabler/Disabler Remade. It was last updated 3/21/20, so the author may update that for Master Mode once we get Journey's End.so, you think you can make this but for master mode?