I believe it’s inevitable that Galvin will move to the back row, either an edge or lock. Once he gets some muscle on his frame, and hence the extra weight, he won’t be quick enough at 5/8 and will have more impact receiving the ball either a little wider or using his passing game as a link player.
As his mobility slows down I think that opposition clubs will work him out at 5/8 without the genuine speed to make him less predictable.
I believe he will hang on to 5/8 as long as he can however, maybe a couple of seasons. He probably is already aware of this but will enjoy having his hands on the ball as much as possible until then.
However I can see him as a Yeo type lock where he could still play like a half and enjoy running the ball as well.