dAGGER: I have overlooked one thing: one could form a blocking position to keep the opponent out of the own yard. This is prevented by the current rule.

It was just a thought, motivated by my dislike of rules with arbitrary numbers like "after 30 moves".

would it make sense to change the winning condition in Halma to the following:

"a player wins if, after the opponent made the first move, all squares of the opponent's yard are occupied (by pieces of any color)"

This makes the 30 / 50-moves rule superfluous. If a player leaves a piece in their yard, it can be used by the opponent to fill up the yard.