The machine we use low beryllium copper (C17500 or C17510) roller
heads for both applying pressure as  well as passing the current. The
material we are welding is steel and as such, the roller heads wear
out very fast, like in a day or two.

Natually, my question is: Is there a commercially viably material that
is signigicantly stronger and harder than C17500 and yet retain the
electrical conductivity at least around 50% IACS?

Check out www.bayworth.com  They are material scientists and can
probably help you with your problem.

As i see it (as an expert in resistance welding) it is most likely
that you are not using the correct pressure, which is causing
excessive wear.
   Any other alloy would also pose the same problem if the pressure is not right.