I propose ships start accelerating when in the vicinity of planets. This acceleration should be modest at range, but steadily more significant at lower altitudes from surface. Ships at a certain altitude are assumed to be “in orbit” but closer their orbit is sai to “decay”.
This would make combat for larger ships somewhat more dangerous near planets. They are accelerated away ever so gradually. An icon appears in the screen “caught in the gravity well, your ship will crash in this orbit in 45 minutes”, signifying how far you are away from the planet and how long it will take until the ship crashes down on the surface. It’s much the same as a tractor beam effect. It should therefore become dangerous to muck about with dreadnaughtys say, 30.000 kilometers from a typical terrestrial world. Vessels with higher propulsion velocity would be able to easily counteract this pull.
Also - clearly anchored structures should be assumed to be impervious to this effect.