Here's a graphic that shows 3 concentric (attempted) rings. This is where I think our core cities should be placed - we should fill out the 1st (or inner) ring, as that gives us the lowest corruption. The 2nd ring (and we can start near the landbridge) should be completed 2nd. Then the 3rd.
Hopefully, this will help as a guide to place our next cities, keeping in mind that we should use our available resources to maximize city growth and production. As a personal opinion, I would feel safer having another city or two around the capital. Not only for being the front lines, but also for production (and less corruption).

Hopefully, this will help as a guide to place our next cities, keeping in mind that we should use our available resources to maximize city growth and production. As a personal opinion, I would feel safer having another city or two around the capital. Not only for being the front lines, but also for production (and less corruption).