Okay, the first one wasn’t great but this one is guaranteed to get about 50 mobs in 30 minutes, it is larger and takes less time to fire up, it is basically two rooms stacked on top of each other and a drowning area, mobs spawn and get swept into a channel which traps and drowns them above a hopper which collects the drops for you, very high spawn rate when built correctly.
Tip: keep all areas where mobs might spawn unwanted lit up otherwise the spawner will not work as well.
Funnily enough, as soon as I switched off peaceful mode for pictures it started working.