When to use external database?

We’re currently building an app with a starting data set of 5000 product pages.

We plan on getting a high amount of user sign ups within the next 3 months. Roughly ~10k users and growing the amount of product pages along the way.

A few questions:

Should we start with an external db before building or just keep everything in the app?

What is a general threshold for when data sets get too big and they start to affect performance?

If we get too large, will it be easy to migrate the internal data set to an external one?

TIA