during splunking trips what is your average iron/minute rate? I would guess around 10 iron/minute sounds right, taking into account the preperation time, travel time, splunk time, etc.
i also think if an iron golem is to be built it should be at least twice as fast as spelunking iron gathering, as spelunking also gives other ores and drops, and also experience, and the benefits of the farm should outweigh its costs. we'll assume killing with a nonlooting sword method, because then you can afk and collect drops. a single village of 16 villiagers will produce 1 iron /87.5 seconds with that method. If we want an iron every 3 seconds we need to build about 30 of these villiages. this works out to about 500+ entities loaded, not unreasonable at all.