I'mma pitch in with some input as-well in the hopes to settle some of the arguments here:
1: Mentality of war
How come Gallia had a stalemate and all they accomplished in 5 years was pour into Magellan?
Because Gallic soldiers were an invading army, they simply followed the orders of a tyrannical outdated government system or suffer severe punishment, they had no intention to invade Sirius, kill so many people while throwing their own lives away for something that happened almost a millennium ago when none of these colonies even existed. They simply have no motivation for this war, while Bretonians are fighting for their very survival, hell Bretonians would make projectile cannons out of trash bins and tachyon weapons out of incandescent light bulbs if it offers them the slightest hope for survival while many Gallic soldiers may harbor secret thoughts of desertion or outright treachery.
2: Gallic war preparation
According to the old Gallic lore, they have spent hundreds of years frantically building up their military might so that they could pour into Sirius and "take revenge" on ALL of the Houses, with that amount of preparation they won't just back off, sign cease fires or settle with what they already conquered UNLESS the monarchy is overthrown by the Council.
I think it may be worth taking in consideration planning an event together with the community at large at a certain time when most involved players will be able and willing to attend where Council and allies will attempt this coup, maybe by having a huge fight in IDF against GRN/GRP and the winning side decides the outcome of the war. (eg. Council wins: Gallic war ends right then and there / GRN wins: Gallic advance continues)
3: An invasion of this scale won't just stop after a few years, it would easily take decades for that to happen, if people in the community are tired of this war and want it to end, perhaps we can inRP skip a few decades and simply write the story of the outcome in the lore without implementing everything into future mod updates, but if we go this route we need to write decades worth of lore for every single faction so it's up to the determination of the community as a whole I think.