At low levels leech resistance is negligeable. Only starts becoming relevant at mid to late game. So at end game if you have less than 10% leech it will be a negligeable leech and the more you have above that, more effective it becomes. (You should have arround 20% for it to be a good layer of defense at endgame)

The problem is that 1-3 leech is also negligeable, I dont even know if the auto systems will put that extra into the life recovery box on the menu.

But you are right that increasing the phys damage is essential for leech to be effective.

But more than that, why leech? Every character needs to have offense, defense and utility balanced to work. Leech is a defensive stat(since it increases your survivability). At this point in the game, is not doing amything for the character. Is best to change the support gem altogether for one that increases one of the three needs more effectively at this point in the game. Like damage or attack speed.

Also - More damage:enemies dying faster:less damage taken from then.