Tracker and integration with an aggregate level national system

I have a few questions regarding setting up multiple tracker programs when an existing national system is implemented.

What is the general consensus with setting up tracker programs in a national level environment – is it preferable to set them up on separate instances?

What about aggregation of data – is it relatively straight forward to aggregate the data from the tracker instance to the national aggregated instance?

Would we need to configure
the process ourselves as an export/import or is this something that can be achieved within DHIS2 automatically?