A while ago, I created an add-on that allows you to create a mob-grinder in one block. I reworked that add-on for 1.16 and future versions. This add-on adds a block that grinds mobs by placing their spawn eggs inside. The interval would understandably take some time in order to balance everything, though this is not an efficient solution to big mob-grinders.
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Added goat and glow squid support for mini-mob grinders.