The core piece of delta loads is of course to find records that changed since the last data transfer.
An easy way to do that is to keep a last changed date with all your records in all tables.
That makes it easy to find those records that have changed.
It's not "pretty" but can - accepting an initial development investment - typically be very easily implemented (especially when the number of tables is rather small).
With MS SQL and Oracle there are also APIs available to do Change Data Capture (CDC) - but my gut feeling is that this would be overkill in this setup.