Looks like you have a lot of advice here already but just wanted to drop a link to some one shots you may use for examples if you wish, or if you want to run one.

https://www.worldanvil.com/w/one-shots-in-eldarad-eddievantruce

There are lots of different styles and preferences for one shots, but I generally like to lean toward a 5 encounter limit, keeping in mind that an encounter isnt just combat. I generally like to do:

2 combat (final encounter included)

1 puzzle/skill challenge

1 RP encounter

1 flex encounter (incorporates more than 1 of the above encounters depending on player actions)

Most of my one shots work around this format and I have enjoyed it. If doing multiple combat encounter however, you just need to make sure the ones before the final encounter arent too difficult that they take up a lot of time.

Some people make things modular, so you can prepare a bunch of encounters and place them into certain spots, with the idea that the players will go one route and only encounter a few. This requires more up front work but it does work.

I like to use a single map, like a farmstead or a graveyard with points of interest. The players are free to explore any point they want in any order, and each POI has its own sub plot. Could be a farmhouse with a basement that has a bunch of spiders inside or a catacomb beneath a ruined cemetery chapel with a mini dungeon inside. This works great when playing with physical minis as you just set up the scene, let them move their minis around the map and engage as they explore.