I’ve gotta agree that a polymer is your best bet here. Something like aluminum may be too soft to resist wear of small gear teeth, and anything metallic is gonna need some lubrication and will have some rattling noise. There’s also expansion and contraction with outdoor temperature variations. With sintered steel, depending on the grade, corrosion with moisture may also be an issue.

There are so many different options with polymers, especially since you’re not expecting high loads on the gears so you’ve gotta be able to find a good one. I think a modern engineered polymer will give you the longevity you’re looking for.

When you say lubrication, I think immediately of nylon. Most grades are self-lubricating. Also, cost-wise, that should be cheaper than something metallic. So I think your suggestion of PA6 GF is a great one, and in fact, PA6 GF is commonly used for gears. You could consider slightly different grades, such as PA66 (but still with the additional glass fiber reinforcement). If you’re able to cast the gears, then cast PA6 (PA6 C) may also provide better mechanical properties due to its higher crystallinity.