I think there's probably a simple solution to this, you just need to change the way the board is organized. I've long found it odd that Powers tend to form a bunch of disconnected bubbles of influence, rather than cohesive empires.
Imagine systems bubbles of influence as cubes, with six sides. Upkeep is removed entirely, and replaced with defensibility. For the purposes of demonstration, I'll just draw four sides, but imagine there's a top and a bottom, too.
Now here, you can see three circles; the green is with a beneficial BGS faction, the yellow is with a neutral BGS faction, and the red is with a negative BGS faction. You have potential systems on all four sides.
Here, we see what happens if the left system is also claimed. Because the two are adjacent and touching, it reinforces the strength of the central system against attack, indicated by the double circle.
Now, a hostile team manages to convert the system to a neutral BGS faction. However, that still leaves the radius large enough to be in contact with the supporting system. Its strength is weakened, but not dramatically.
But what if they manage to turn it completely against the Power? Suddenly things get a great deal worse. Not only is the power substantially more vulnerable by default, but the radius decreases enough that it loses contact with supporting systems, leaving it perilously vulnerable to attack!
Now, lets go back to the starting condition again. You've got a system with a strong BGS presence, supported by an adjacent system. Normally, a very challenging target to attack! But the enemy, knowing this, chooses to move directly adjacent to the system, weakening their relative presence. As you can see, at the contact point, the reinforcement from the other adjacent system is removed, leaving only the default defenses.
This makes for an easily understandable yet relatively complex system of move and counter-move, trying to create vulnerabilities or leverage strengths.
Lastly, what about that little star on the side? In the current system, taking a star so nearby like that would be a terrible decision, as it would be worth very little AND cost you a great deal. Lets take a look.
As you can see, the result is...absolutely nothing. Because upkeep has been removed, the star costs nothing, but it also GIVES nothing. However, in taking it, the Power opens itself up to being attacked there, for no benefit, and doubles the amount of fortification they may need to do to keep both systems. A wise Power would let that system lapse. (A potential way it could work is, if an attacking power causes the system to fall, they get all the Fortification that was dedicated to that system. This handily avoids the potential for collusion, as well.)
With upkeep and CC and overheads largely decoupled from system ownership, there would be no need for convoluted math around losing systems, they could just be fought over directly. Power size would be kept in check by the opposing Powers, which would be rewarded for causing other factions to lose a system, which should probably be weighted against the larger powers. If the smallest power causes the largest power to lose a system, then they should get a huge dose of CC to do with as they please, ideally for fun Power-related stuff like Fleets or Invasions or Assassinations or whatever else.
At this point each individual point of conflict is relatively simple, with at most six systems to consider for any given system, plus BGS states, but since each system would also interlink with each other system in the Power, there would also be great potential for complexity. After all, losing one system could cause a huge instability in a large power, so it would need to be built with redundancy in mind, all while avoiding unnecessary fortification.
That's what's needed; a mix of simplicity and complexity. Simple enough for the average player to understand instantly, complex enough to fight over for years.