Query requires 'Update statistics' very often in SQL Server
Data is bulk inserted into these tables in the order of ~ 200k rows per day... out of ~5 million rows


That's 200k new rows that do not have any stats for, so querying for those values will make the optimizer come up with scans because of the outdated stats do not inform it that the new values are in the range sampled.  Force a stats update after the bulk insert. 

Not sure autostats can help given that 200k inserts fall bellow the 5 million 20% cadinality trigger, see Statistical maintenance functionality (autostats) in SQL Server.