Is the Neptune 55 bright enough for direct sun?

No. At 500 nits it is designed for shaded or covered areas. In direct sunlight the picture will look washed out; full-sun outdoor TVs typically run far brighter.

Can it stay outside in rain and winter?

It is rated IP54, which covers dust and water spray, and it operates from -22°F to 122°F. It is not rated for full immersion or heavy direct rain, so a covered location is best.

Does it need a streaming stick?

No. It runs Google TV with the Play Store, so you install streaming apps directly on the set.

The TV and a remote control. Mounting hardware and a stand are not listed.

Does it support Dolby Vision?

Yes. Dolby Vision HDR is supported alongside the 4K HDR panel.

How does it compare with brighter outdoor TVs?

It gives up peak brightness for a shaded-space design. If your wall gets direct sun, a brighter full-sun model is the better fit; for a porch or pergola it is the more sensible match.