Being based in Amsterdam is the key here. You'll always have that "home market premium" regardless of where you are.

It's the same reason why I used to fly British Airways often when I was based in Paris. And not a single time since moving to the UK

If you're keen to stay with ST you'll need to look at DUS as a base. Otherwise lufthansa group does indeed seem like a good choice. (Unfortunate that their loyalty program is garbage)