Unet, Mirror, Dots ..... Which one to use for first attempt at a multiplayer game?

If you haven’t checked out this thread, take a look:
https://discussions.unity.com/t/727663

Unet was deprecated over a year ago, but effectively it has been deprecated since at least 2016, as Unity had basically stopped developing for its high level API at that point and had 1 developer working on occasional bug fixes for the low level API. But Mirror has lots of bug fixes and active support by the developer, so I wouldn’t be concerned about it going away so long as the developer’s network related assets on the store remain popular.

On your last question, ask yourself if you want to host a dedicated server for these matches. There’s pros/cons to doing so. The answer will help you decide between those two API’s.