I think I'd prefer action-to-salvage (so you can still hide in the fields without filling your holds up) and entire field is salvaged in one go.
I'd love coalescing fields if they touch, and I also think bigger fields should take LONGER to salvage, with you sitting there watching a bar on the main map.
If you move before it finishes or an enemy fleet contacts you or some other interruption, then the cargo screen immediately pops up with a partial result much like we have now, so you can choose to dump the metals in pods and run, or whatever.
The field should then be reduced in size appropriately, so you could come back if you're interrupted - but if you let it finish, the field is removed as you have scavenged everything of value.