Why will nobody do a rebuild? Option, find a good shop, have them pull the trans, then crate it to someone (who knows subarus), they swap the gear out, then crate it back, that shop slaps it back up and you are good to go. I just did this. Much cheaper. To crate the trans from Houston to Dallas ran me $200 each way. Just a thought -
Otherwise, you are right about replacing the clutch - great time to do it. Also, swap out the clutch fork and pivot ball with the Versus stuff (https://www.verus-engineering.com/product-page/fr-s-brz-gt86-billet-clutch-fork-pivot https://www.verus-engineering.com/product-page/forged-clutch-fork-2015-wrx)
Sorry about your luck. Like I said, this just happened to me - blew up 3rd gear (broke some teeth)