High voltage input/Low voltage, low current power supply
Galvanic isolation would be a requirement from such a high input voltage, especially for something coming from a utility grid. I would not use a 'dropper', or try to design a 600V input AC-DC flyback on the first go as you propose. (Let's be honest. If you're asking here, chances are you haven't yet developed the skillset to make a safe off-the-line switcher design at this high voltage.)

Instead, I'd play it safe and use an appropriately-rated step-down transformer with a primary wired across two phases or phase-to-neutral, then use a conventional low-voltage DC design in the secondary.

Such a transformer would be quite small as the secondary windings only need to deliver, say, 1.5VA tops, and it could be pretty well-spaced primary to secondary (end-to-end coils rather than overlapping ones). A transformer vendor could deliver this to you, fully hi-pot tested and ULR compliant.