Just got done building it, and I'm worried I built it too low to the ground.
Functional, yes; but efficiency will be reduced (particularly at night).
Assuming you light (or otherwise spawnproof) everything within 128 blocks of your AFK spot this is correctable. [But it would probably be faster just to rebuild farther up.]
If you are in 1.13 etc., the anomolous spawning rules for MobB will also impact your nighttime efficiency unless the spawnable volume is blocked [20-34 blocks above the player and up to nine (taxicab) horizontally], or you stay under a solid block roof.
Rollback Post to RevisionRollBack
WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.