Grats on deadmine ! I learned a lot boxing that instance (for instance that I need to turn around with my tank so I can easily see what my slaves are doing)

I think if you set your assist to /assist focus (without trying to deal with focus less or leaderless) you'll find you have no lag issue - that's what I do and even if my main die, I can continue to target mobs (though typically in instance if your tank dies it's likely a wipe if you can't finish the mob quickly so optimizing for loss of leader which is 1% of the time at detriment of 99% case seems wasteful for PVE to me)