Is this TV good for a dark room?

Not really. It has no local dimming, so blacks look gray and backlight bleed shows in dark scenes. It does better in a bright room or for daytime viewing.

Does it support Dolby Vision or 120Hz gaming?

No. It handles HDR10 only and has a 60Hz panel, so it lacks Dolby Vision and the high-frame-rate modes that consoles like the PS5 and Xbox Series X can use.

Do I need a soundbar?

Most owners should plan on one. The built-in speakers are basic and sound thin on movies and music.

What is the difference from the F20 or F30 series?

The F50 line sits above the entry F20 sets, mainly in size options and features. All are budget Fire TVs without local dimming, so the main reason to pick the F50 is the 85-inch screen.

Can I control it with Alexa?

Yes. The included voice remote works with Alexa for apps, inputs and basic commands, and the TV fits into an Alexa smart home.

Does it have enough HDMI ports for a console and soundbar?

It has a standard set of HDMI inputs plus optical audio, which is enough for a console, a streaming stick and a soundbar, though you may need a switch if you add more devices.