It doesn't matter what your macro is, when you change targets the slaves will be on the old target... for roughly double your latency.
It is just how the game works.

The main/master changes targets.
The target change goes to the server.
And comes from the server to the alt/slave toons.

You can set a focus target.
And switch from the current target to the focus immediately.
Because each of the toons already knows who their focus is.

You can include [nodead] in a macro if you want.
This should prevent you from casting at a corpse.
It won't speed up the time it takes the server to notify you of a target change.

Player = Current Toon
Party 1 = Party Leader (if not the Leader) or F2 Target (if you are the leader).
Party 2 = F3 target
Party 3 = F4 target
Party 4 = F5 target.

You can use a variety of assist methods.

/cast [target=Party1Target] Spell

/assist [target=Party1]
/cast Whatever

/assist [target=Focus]
/cast Whatever

/castsequence [target=FocusTarget] Whatever

Or a target based approach:
/target [target=ToonA,exists,nodead][target=ToonB,exists,nodead][target=ToonC,exists,nodead]...
And...
/cast [target=TargetTarget] Whatever