We need quantum entanglement based communication. Maybe without full collapse, using weak measurements, like Alice continuously broadcasts a "retrocausal carrier wave" by sequencing planned future post-selection measurements on her entangled qubits, which backward-propagates through time-symmetric quantum evolution to create detectable perturbations in the present states, biasing Bob's qubits away from pure randomness to encode message patterns.

Both parties perform weak measurements on their qubits to extract these subtle signals without collapsing the entanglement, preserving high coherence across the stream. A quantum Maxwell's demon (e.g. many experiments but can be done: https://pubmed.ncbi.nlm.nih.gov/30185956/) then adaptively selects the strongest perturbations from the wave, filters out noise, and feeds them into error correction to reliably decode and amplify the full message.

The problem is each relay needs its own power source so it's not going to be as light and small as you would like. Solar power doesn't work very well outside of the solar system, or even really in the outer solar system.

On the plus side your big probe could push off of the small probe to give itself a further boost, also necessary because otherwise the small probes need thrusters to slow themselves to a stop.

Sure, drop one repeater every light-day. 1500 of them. Each one will need fuel to decelerate enough to remain in place.

You can't leave anything behind. That would need to be accelerated to 50,000 km/h or have even bigger rockets than launched Voyager in the first place.