IMTU giant worms or bugs are a common Import to a new world. Genetically modified to harvest resources of a given planet and create vast tunnels and with their nests they are the most basic line of defense against orbital bombardment.
A reinforced underground tunnel system would be the easiest to make a home out of. As well as their organic tunnel system can confuse ground penetrating active sensors.
Artificial or Enhanced Belts.
Belts serve a very powerful defensive advantage - cover and concealment. Particularly protection from orbital bombardment. As each chunk of rock can be enhanced and given spoofers it can potentially create an Information Assymetric buffer for defenders.
Artificial Dust Clouds or Rings.
They give more and more concealment especially when there are belts.
Semi-Mobile Space Ports and Stations.
Ports that move around is a good standard, especially given the Evasion and Pursuit Game theory principles. This is truer for military installations and various defensive measures.
Advanced Defense Probes.
Early warning Probes that give out maximum data read out of incoming traffic and threats.
The more efficient and massively productive economies of such Scifi settings would have a high allocation for defense. Contracts to litter rocks on belts and rings, and creating various redundant installations underground. Contracts for Spoofers and Electronic Warfare Countermeasures, as well as sensor redundancies.
this was common practice in medieval times: the merchant fleet serves in the Naval activities. Trade, Security and War. This gives economic flexibility as well as much better economies of scale. ,
Q-Ships and Doctrine of Deception. The time proven trick is to hide fighting power behind standardization of design. When Warships pretend to be Merchant Ships and vice versa, States can more efficiently use its numbers. Long traditions of blending what is good for a warship being good for a merchant ship can mean easy retrofitting to serve as combat or trade ship.
Underground or Submerged Fleets.
Planets are great at concealment, while it would be standard to always set a fleet's reserve as far away from observations of "allowed" traffic routes as well as Optimal Advanced Probe Barrages (sending out a ton of probes into a system as a first move), one of the hardest to detect are ships hidden in gravity permitting worlds. Even if the escape would take a number of hours, such concealment and possibility of it is great deterrent.
Defensive Political Will
Economies of peoples with strong memory of war and what happened before is one of the best kind of ways to have defensive measures achieve political will.
The compound Interest of Defensive Productivity.
Using various game systems and compound interest its becomes easy to imagine how much this "rainy day fund" can grow. Start with the fund and budget and set a compound interest over time.
There are many unintended consequences to a defensive mindset.
- An Awesome Inflation Sink. Measures set aside for defense is "out" of the economy and circulation. Finance AIs and Policy Makers can make it as a way to control inflation as well as to motivate greater and greater productivity.
- Amassing a Fleet that can be used for Aggression.
- More extensive Bio-mods for survival or aggressive population diaspora (Zerg strategy)
- Paranoia and Fearfulness. Possible Isolation.
- Failing to see a crucial weakness. Tunneling mindset is natural when using fear based motivation. A crucial weakness typically emerges and there is a rush to close this gap when spotted. Its possible to have such peoples all die off and leave behind massive reserves of technology and supplies.
- A bleak culture of death and obsession over it. all the joy is sucked out of enterprise.
- Void Nomads - peoples whose home systems were destroyed because they focused on the Merchant fleet aspect. Or this was their strategy all along, to just keep moving.