Sacred Haven of Debauchery

In general, if you're playing on a high difficulty level and are afraid of being overwhelmed, it sometimes makes sense to befriend one of the strong empires and become their vassal. And then, wait for the right moment to not only break free but also to seize part of the former suzerain's systems.

I was playing on normal difficulty :o And even in the best-case scenario, the approval rating was -50, and usually -300 to -500. The story of this conflict is actually quite amusing:
I successfully enslaved two races and started gradually conquering the blue ones (those surrounded by a purple ring). They were allied with the red ones (those humans with the weak fleet) and the blue ones at the top. I had already been at war with the latter and had an approval rating of -500, but before that, I had also been at war with the green ones and mercilessly exterminated them on a planet with a temperate climate. So, I thought that if I tried to conquer the ones surrounded, even though they had almost no army, the blue and green ones would team up against me. And the mushroom-federation (purple) would help too. I managed to befriend the red one, he invited me to join the war, and I agreed. We fought against the gray and green ones, but the red one wanted more and more and refused to make peace, even though the score was already 40. Then, suddenly, the mushrooms declared war on me, even though I hadn't done anything to them, maybe just declared them rivals. And then the blue ones decided that this was their chance. The irony is that by helping the red one, I was hoping to reconcile with the blue one and calmly crush the green one, who had no good alliances, but instead, I provoked both the blue one and the mushrooms.