I'm pretty sure IME updates take care of it just fine... when I woke up this morning and tried out Microsoft's Japanese IME after finding out the new era name,

I'm pretty sure IME updates take care of it just fine... when I woke up this morning and tried out Microsoft's Japanese IME after finding out the new era name, れいわ became 令和 on the first try - no messing around with the "misconversion" data necessary. Despite that, Gboard's Japanese version hasn't caught up yet, but I imagine that'll be updated fairly quickly, either manually or OTA.