Using the unloader with a locking rail on top, if the tank cannot be emptied the entire way, a redstone pulse is emitted by the unloader. Unsure if this is intended or not.

I think this is also on you how complex you want the menu to be. I feel like the behaviour: work is done (unloading is finished, but minecart has some amount inside) emit signal - has some use cases.
On the other hand, if you transport fluid, energy or items just from A to B, if B gets filled up to the max, this behaviour right now starts to send the minecart between A and B non-stop. While I just want to make it stay on B until the minecart is empty and afterwards can get refilled on A. (Note: While writing, this might be also a thing with loader - haven't tested, I always had enough supply to fill the minecart)
I think this second scenario is much more used and also is the expectation (at least it was mine). So maybe add a button inside for changing between them? But how complex you want the unloader to be, that is your decision.
I'm sorry for the long ramble.

This is big. I hate it. Any idea for a workaround? I have no idea how to read the minecart content to use redstone logic to somehow make it work.

I'm going to look into this. Is the desired behavior having the pulse only emit when the minecart is completely empty?

@Bellwar thanks for the great summary, it is exactly what I was thinking. My thoughts were that it should work like the items, i.e. the unloader doesn't emit a pulse until the cart is completely void of fluid.

Alright, I've fixed this issue on my end, and will be included in the next patch here in a few days.