A disagreement over the PostgreSQL trademark

You don't have to sue. That's the entire thesis here. You're going to keep going around this loop because you're convinced of a myth.

If you subsequently file suit, the court won't "conclude you don't care". You filed a lawsuit, so obviously you care.

Estoppel means that you can't win if your argument is that behaviour you previously ignored is suddenly now something you cared about and the court should retrospectively fix it. Time's arrow doesn't work that way, and the principle of estoppel reflects that. But Estoppel is deliberately very limited, it's just to prevent the sort of obvious unfairness of e.g. waiting until somebody's use of your mark makes them a nice profit and then swooping in to claim it for your own.