Can we? Yes. Should we do it right now? That's debatable.

The question is how much this would cost vs getting a two weeks offline every 26 months?

These two weeks do not create any additional requirements (you already have to make sure the probes can survive for a few weeks without comms), science does not fully stops during these two weeks. And it gives an opportunity to do long duration maintenance on the ground segment.

Frankly, there is little need to spend >$100M for such relays satellites until we actually have a permanent human presence on Mars.

The problem you face with that idea is that the satellites will have to have enough power to retransmit signals.

While the Mars > L3/L4 > Earth route is not much of an issue as the large receivers on earth can deal with a small power output at Lagrange. A signal moving in the other direction will have to be quite powerful to reach the small receivers on the Mars end.

There's no pressing time urgency. It's okay to wait 2 weeks so yeah we could but it's an enormous expense and not worth it

We use very large radio dishes to communicate with craft at Mars, so that the spacecraft can use smaller dishes and less power. In order to add a relay at L4/L5, that relay would also need very large dishes and high power usage to reach the craft at Mars. Probably larger than anything we have in earth orbit today.