Conveyors are 7/8 of a block so mobs don't spawn on them, as for cursed earth ignores player distant, normal mob spawning is you can't be within 20 blocks, 32 blocks will spawn but have a very small chance to de spawn, 128 blocks(something around there) they won't spawn and if your that far away they automatically absolutely de spawn, your best choice would be to have a normal drop with cursed earth and conveyers, drop so they have half a heart at least and then conveyer them to a autonomous activator with a extra utilities sword made of unstable ingots since it doesn't take damage(or a stick just make sure its on hitting mode) then collect the mob essence with a vacuum hopper and pump it into a rail craft tank and have that either go to a openblocks tank or a xpshower, with a lever or else it goes infinitely, also have essence berry collecting in the backround so if your ever for some reason out of xp, then you have a quick backup
From the wolf master!! *insert wolf sound here*