I suppose this is a hot take, but I'd never intentionally select a closed source paid database or programming language. Your data is the most valuable thing you have. The idea that you'd lock yourself into a contract with a third party is extremely risky.

For example, I've never seen a product on Oracle that didn't want to migrate off, but every one has tightly coupled everything Oracle so it's nearly impossible. Why start with Oracle in the first place? Just stay away from paid databases, they are always the wrong decision. It's a tax on people who think they need something special, when at most they just need to hire experts in an open source database. It'll be much much cheaper to just hire talent.

Meanwhile I've done two major database shifts in my career, and you are correct, keeping to ANSI standard SQL is extremely important. If you're on a project that isn't disciplined about that, chances are they are undisciplined about so many other things the whole project is a mess that'll be gone in ten years anyway. I know so few projects that have survived more than fifteen years without calls for a "rewrite". Those few projects have been extremely disciplined about 50% of all effort is tech debt repayment, open source everything, and continuous modernization.