My Suggestions: -
What would be the best approach to bring those industrial data into TX platform? Edge SDK or Kepware? -- Kepware is really good for Industrial data handling and communication with TWX Platform. Using SDKs, you might have to spend good amount of effort & time to implement and test functionality.
How would I manage those things in TX and whether should I go with timers/subscription if any update or req manipulation after every data push? -- Yes timer/subscription should solve your purpose.
What database I can go? PostgreSQL will drop performance after 500 GB. -- PostgreSQL should be cost effective and can use purging/vacuum database option to squeeze space. Otherwise, you can also evaluate Cassandra DB.
Can we use connection servers? Pl suggest how many and how can we implement? Yes, Connection server can be an option if you are okay with using ThingWorx Always On protocol for data communication. I believe you can 2 CS and review below hardware recommendations.
Note: These estimate should not be used directly in Production without through review and confirmation.