I believe that you're not considering a huge set of costs. You'll have a huge amount of lost learning all the way from the Help Desk, through the Developers, up through the DBAs, and even with Managers and PMs. You'll also have to consider any impact you might have on your applications, backup systems, replication systems, DR systems, etc, etc, ad infinitum. True "Portability" between unlike RDBMSs simply does not exist even if your ORMs can handle it.

And, to be blunt, you shouldn't need 40 core to support "about 200 users" even on the worst day and changing RDBMSs isn't going to change that. Actually it might... depending on what you change you, you may, indeed, need 40 core depending on the condition of your code. 😉

Whatever you decide, be very, very careful to fully identify ALL the hidden costs because they can be huge, indeed. Even something as simple as not having as many forums to go to can have a major impact in development costs (never mind the lost learning that I previously identified).