This is all very fair and useful feedback, thank you.
I originally did intend to have set objects/installations that you'd have to destroy for the fourth mission, but I simply ran out of time for this DD. I'm going to be redoing a bunch of levels and the mission's structure since all of them are honestly placeholders right now. There's going to be different missions other than search and destroy/shooting of course - defending and escorting missions will be included as well.
I'm not too keen on adding indestructible terrain since I feel it takes a lot of the oomph out of the gunplay and weapons if they hit objects and don't react, but I understand that this can make the levels feel maybe too open. I'll see if I can include enemies that also demolish terrain when the player is close and don't hold back firing even if they're behind cover. I realize now the player maybe has too much advantage by being the only one doing that.