D&D 5E Input
Hello!
Congratulations on trying something new! Those are interesting questions. I am not certain how typical my responses would be in the larger community, but since you asked...
Level: Your method is my preferred one. While not averse to starting at a higher level, I do much much prefer starting from the bottom. It feels like a greater accomplishment, and the play style is much different than you will experience later. And some of those scares at the lowest levels can be reflected in the personality and play style of the character that they might not have considered. So, your method is most enjoyable to me.
Setting: I love, love, love Mystara. Greyhawk also interests me.
But on the other hand, Eberron is a solid no for me, and Ravnica leaves me cold.
The other settings fall somewhere between the two.
A homebrew setting would also fall somewhere between these two extremes. If it was closer to the ones that interest me, wonderful. The closer it is to the others, the more likely to pass on it.
What draws me to the settings I like is that I have some knowledge of the world. It seems like I can make more of a mark on it, if I have an idea of the parameters, and they worlds seem more open to changes. And the fangirl in me can be excited when I run into something I know as a player- an encounter with Vecna or Azalin would chill me far more than an encounter with a homebrew lich I have never heard of. Meeting Mordenkainen or Etienne d'Amberville means far more to me than meeting the homebrew archmage. You meet legends.
What I like about homebrew is the feeling of exploration. Since it is starting from complete scratch, everything is novel. The trick is to keep things interesting once more of the world is known.
This is a long way of saying, it depends on the setting. Homebrew is exciting, but a pre-made can have higher highs and lower lows.
Style: In general, sandbox is much more fulfilling. It is a world, and it is disappointing to feel boxed in or railroaded. So if it is all open and there are many choices and hooks, it feels like we have true agency to make our mark as we desire. This also goes well with premade settings- if the players are familiar with the world, then they can have many more self-direction to explore and come up with ideas of things to do.
~However~... sandbox can be interminable for an online group. If it takes the average group several days to decide to open a dungeon door, sandbox is generally not for that group. Online, it really seems a clear-cut story works better.
So, in person, sandbox. Online, in a solo game or a game with a very small group, sandbox. Online, in a typical adventuring group, a clear cut story is preferable.
Combat: My preference is just to play it out, despite the length of time it takes. For some people, the challenge of combat is the attraction of the game. I might enjoy the RP with other PCs and NPCs most, but I would not want to short-change someone who enjoys the mental chess of a battle over the roleplay.
But still, do not delay things either. Stick to the regular posting deadlines to keep things moving, and move to the next turn when it is reached. Players who do not respond within that can have their characters either do some preset default action, or do nothing, or have some logical course of action chosen by the DM.
Hopefully it all goes well!