Just found this note regarding transmog changes coming in Dragonflight.
“…coming with Dragonflight is a change that will see transmogs be more useful and with less grind, meaning players can spend more time showing off their awesome unlocks instead. Per World of Warcraft YouTuber MrGM (per PCGamesN), transmogs will now unlock for alt characters as well once you’ve earned them, giving you access to them regardless of which toon you’re playing.”