Alright, well here’s a separate thought:
Have a toggle that allows for allocation of industry into two categories, defensive and offensive. Offensive industry creates ships, defensive industry creates static minefields which can be cleared or decay. A similar mechanic was seen in the game Stars! if you ever played it… without ships to constantly re-lay mines the fields would decay.
In this case, much like offensive industry, defensive industry would have a production rate but also have a decay rate. Minefields would stabilize at a predetermined size based off of the amount of industry supporting them. This size would be augmented by manufacturing in some fashion. For example the mine cap would be y= i * ( 5m + 15). So at i=1 and m3 it would be 30 mines, but at m1 it would only be 20.
So imagine a 2 industry planet with M1, with 1 industry tasked to defense it stabilizes at an ‘effect 20’ minefield (kills 20 invaders). With 2 industry tasked for defense it stabilizes at an effect 40 minefield. The rate of generation would be exactly the same as with offensive industry.
Let’s say I switch industry over to offense with my effect 40 minefield in place. Well the rate of decay should be the inverse of the growth rate for that same level of industry until the next effect level is met: So from 40 to 20 it would decay at a 2 industry rate, and then from 20 to 0 it would decay at a 1 industry rate. Otherwise you could program it so that the rates simply have stable points and are constantly fighting eachother rather than being piecewise defined… though this would be far less predictable and would need significant play testing to refine.
The caveat with this system is that it would likely only be used on edge systems where attacks are expected because of the loss of offensive ship generation, and only when the threat of attack is very real, since the minefields would decay and constant use would put a player well behind in terms of offensive ability.
Finally the advantage given to taking worlds still exists, no industry is lost.