Expandable side room iron farm

A cheap iron farm for three villagers and one zombie built around a central spawn area. The zombie sits in a boat below the villager room, golems spawn on the platform and drop into lava over the hoppers. More villager rooms can be added on the other sides of the same collection point, about 10 blocks apart. Java only.

Wandering trader llamas spit at the zombie and kill it, and this is the most reported way the farm dies. One builder solved it by dropping the boat and trapping the zombie under trapdoors on a redstone clock instead, and confirmed his zombies survived an AFK session afterwards.
The fence gate has to be crimson or warped. Any other wood burns from the lava, which the creator confirms with "use nether wood, it will fix your problem".
Golems spawn too far out and never see the zombie. Fence the spawn area in, or put a path block border around it so they cannot spawn outside it at all.
If golems stand on the edge instead of falling, put trapdoors around the edges and close them.
Put buttons on the floor of the zombie room so a golem cannot spawn in there and kill him.
Golems swim up in the lava and some iron burns. Soul campfires on the hoppers kill them without burning the drops.