Do you mean any story (comics and novels), or are you specifically talking about novels? 'cause I imagine it's a little different for each, but especially a difficult slog for novels.
Generally, though, an attractive thumbnail and interesting description are the best place to start in terms of getting people in the door. Then the quality of the writing or art will keep them coming back.
Luck, trends, and promotion also obviously play their part, though. And sometimes they play a BIG part.
Mm, just part of building an audience. 500 subs won't get you much in terms of ad rev or anything. I think it's just about the next relevant goal. You always want to focus on a number that feels attainable for where you're at, though. Like 25 subs is a huge achievement. 25 individual people are reading something you made! And then you aim for 50, and then 100, and you keep plugging along. Your goals might shift depending upon your long term interest (hobby, fun, professional, popular, etc)
Nah, sub for sub is not the best approach, as it has a high chance of people subbing but not actually reading or interacting with the story. It's the regular interaction that helps boost a story into trending and such. An inactive sub might weigh you down instead. Though it's not terrible to work with a few other small authors when you're first getting off the ground. ^^
Uh, probably? It varies. But assuming you mean premium, they get a lot more promotion by tapas on the front page, so they have an advantage to gaining readership. I've only really read one premium novel that I was gaga for.