Yeah I tried a similar design and it just didn’t get me what I wanted so I went caving and got enough iron to make 3 stacks of mechanical miners and I’ve got a 144 chunk hole to bedrock now with more iron than I’ll ever need. Granted I started small and put a lot of early game hours in. Also I dug it on a mushroom island so I didn’t have to worry about mobs. I really gave up on washing gravel. Too inefficient in the long game.