The vanilla redstone circuit for unloaders is bugged in 1.16.5 and modded players don't like using redstone much anyway.

Minecarts are cool and we should be using them a lot more. If we had something like this I'm sure we would.

How about this: A powered rail that detects if a minecart chest is empty or full.

It has a normal and an inverted mode.

In normal mode it powers when the minecart chest is full, and in inverted mode it powers when the chest is empty.