A single session should be doable with one or two pages. I like the style the Mausritter community uses a lot. For an example, see the Stumpsville adventure in the base game.

A larger good example is The Waking of Willowby Hall. Multiple pages describing every room in a big mansion.

• There should be a single paragraph intro which can be given to players in advance (so no spoilers!).
• An image is great to convey the mood and style.
• A map is usually useful but think broadly. The map of Stumpsville is more like a picture from the side. Still good enough to say "you are here in now".
• Offering multiple plot hooks is great to embed it into a larger campaign.
• Lots of bullet point lists and bold text parts to optimize orientation during play.
• To avoid going "off rails", just state a clear goal. If they go "off rails" then it is a short session.
• Something I miss in many adventures (especially if sandbox style modules) is a sense of urgency. Work some kind of timer into your adventure. Willowby Hall has an escalation mechanism built in which increases danger essentially because time passes. In other words, don't just "rescue the princess" but also "she gets sacrificed in three hours" and it should be possible to reach her too late.

What I don't need is a showdown scene. In general, I prefer "adventure sites" rather than "adventure stories".

I highly recommend you look up a 3rd party D&D 5e adventure called "The Secret of Skyhorn Lighthouse". You can get the PDF for free. The author does an absolutely incredible job organizing information on the pages in a way that perfectly captures just the stuff needed for the adventure and nothing extraneous. IMO it's the best organized adventure I've ever run.

Every scene is limited to just one page. With clear explanations of each important NPC one might meet in that scene, what the dramatic tension should be when the party is on that scene, etc.

If I could have every adventure I ever run for the rest of my life follow one format, it would be the format of that adventure.

What's the info you would be looking for when running published adventures?

Include multiple paths through the module with multiple resolutions.

Cyberpunk RED has a neat way of structuring adventures that does this well: there are a series of scenes described, that branch to other scenes. They're categorized as 'Dev' for plot development, 'Cliff' for items that increase tension, and 'Climax' for scenes that resolve arcs.

"Agents of Desire" in the Street Stories sourcebook is an excellent example.

I find I like adventures that are more a bunch of toolboxes that I can pull out when I need them as I’m running the adventure. They give a full background of what happened behind-the-scenes that I can choose to reveal through player interactions with the world or characters. The rest it gives a range of options for how to treat some common situations, but doesn’t assume to know what the players will do. It’s a hard balance to find sometimes though, if it depends entirely on the PCs doing a specific thing just to even get into the adventure in the first place.

I don’t really like when they just assume X will happen, which will lead to Y, then Z. You don’t really have to worry about a game going “off the rails” if they’re not really there to begin with. I like interesting and colorful descriptive details added about anything, characters, rooms, buildings, environments, monsters, whatever. Even if it’s just hinting at things, anything that gives me something to go off of and flesh the world/characters with.

The modules I like have:

• A DM map with a bunch of numbers on it, and text sections corresponding to the numbers detailing an encounter in a certain area. I personally skim these ahead of time, to know which parts to read out to the players when/if they get there. These should be traps (with exposition hints), puzzles, and combats.
• A hook, escalation with two options, and resolution, all encompassing a possible plot. TBH, this should be something that a DM can discard and replace with their own plot, if they have the inspiration and energy to do so. But if they don't, then your prewritten plot is there for their use. This is required reading either way, to know what's important (or what to replace).
• Some NPCs that have basic goals and motivations, for the DM to RP if the players find them (or need a push.} You don't want more than a paragraph or two for each, because all the extra details should be ad-libbed anyway. Motivation is key tho--why are they there, what do they want, and where their lines lie. Two one-liners from a Background table along with an alignment can usually cover most of that, TBH. Limit the required reading to 3-ish named NPCs per session, or less, with fewer introduced in subsequent areas of the module.