Yes, there are endless reinforcements. You want to rush the gate controls. Even on Etermes difficulty it is possible to activate them on turn 4. I once did it on turn 2 (on Hero). This is another example why Movement might be the most important stat. You will never have to face the Scylla. I recommend bringing 1 or 2 Archangel Rocket launchers: 2 shots are enough (3 on Etermes) to disable an Explosive Bomb Chiron’s abdomen in the final room.

So once I hit the gate controls, enemies stop spawning in the first area before the gate?
There's little point in killing anything, just run if it's not in your way?

There is no stop in reinforcements. Just RUN. BTW, on Etermes reinforcements spawn with 4, fully usable AP. Better pray that not a SR Triton is among them.

I killed a few of it's eyes and the receptacle died even though it still had like 80% health

4 eyes on Legend, 5 on Etermes. Isn’t it an improvement?

I dont know... On all my other 10 playthroughs I killed Receptacle by taking its health down to 0..

You mean all I had to ever do was disable its eyes?

Yes. This is part of the final mission rework.