You're probably overthinking the realism. you do have prior art in all aspects though Zero Point - Skies of Arcadia Propeller - Final fantasy balloon with co

You're probably overthinking the realism. you do have prior art in all aspects though Zero Point - Skies of Arcadia Propeller - Final fantasy balloon with combat - Red alert 2 Kirov Airship will kinda depend on if this is 3d or not. if you're first person just shooting at the balloon portion, then yeah. go zero-point since profile will matter greatly. If this is an isometric or top down kinda game then like red alert you're just shooting a target not a piece of a target and you can just ignore damage to lifting apparatus (propellers or balloons) without breaking immersion.