L3

Did you mean L5? L3 is always in line with the sun, so it doesn't seem like it would be useful for communication.

A signal moving in the other direction will have to be quite powerful to reach the small receivers on the Mars end.

Would it be easier to have a separate satellite for each direction, one at the Earth-Sun L4 point, and one at the Mars-Sun L4 point? Could we get a large enough dish to the Earth-Sun L4 point?

Alternatively, could we use lasers instead of radio? The SpaceX Starlink satellites have laser inter-links, and NASA just sent up the ILLUMA-T payload to the ISS last week.