Avoid Vacuum (to prevent wraparound) on read-only databases
from your description , you actually need this process or else your old data will effectively become future data that you can’t access (wraparound)

You have 2 options here:


Run the vacuum yourself in non-peak hours to better control load
Reduce the number of transactions 
(E.g. if you load data hourly you’ll have 24 transactions, each one loading just 41k persons )