Import / Aggregation DB - with 50 Million Records - Best Practice?
I'm not sure why you would do two different servers.

Your front end can be cached with Blitz / NGINX which will take a load off the server and admin.

How often are you aggregating the data?

I've just imported 10k records from a source and I would recommend writing your own module to handle the imports as commands. From there you could set up a cron which would run at a scheduled interval. These commands wouldn't run via the web server so you wouldn't be restricted to memory / timeout limits.