Minecraft - Guardian farm low spawn rates

Solution 1:

Do not underestimate the importance of lighting the caves! If there are many valid spawn places outside the farm, the mobs will spawn there and stay there for a long time eating the mob cap. Even a small number of valid spawning places outside the farm can make a big difference, because mobs outside of the farm stay for 30+ seconds and mobs in the farm die within 5 or so seconds. For example, if the farm has roughly 2000 spawning spaces and the caves have roughly 300 valid spawning spaces, the farm will work only on HALF of its capacity or even less.

So, you need to light the caves and make sure you do a thorough job of it. You'll be pleased with the results. Good luck!