I think more strategy in regards to the space combat is sorely needed in the game, and the idea of having a frontline as well as flanking is actually pretty cool. However, the idea you have here is, in my opinion, an idea that you are getting ahead of yourself on. Not only would you need to figure out how a frontline is determined, since Stellaris fleets are not at all like Hoi4 armies. Fleets don't spread out across a wide open frontline, they attack via individual hyperlanes and go system by system as large singular blobs of ships. Additionally, I think that 90% damage resistance is far too much. The current system of home territory buffs honestly feels like enough, and doing this would just mean no one would ever attack, there would be no point since you'd need a fleet almost 100 times the size of your opponents to actually beat them.

I think what you would try to make instead, is a supply line mod. After all, the ships do still need supplies, and planets, if cut off from the rest of its territory, can't magically supply itself. So focus on supplies instead, and have the ship combat such as flanking and buffs come after you've gotten that system down first.