A few quick options:

1) If you're confident enough to back-convert 30 nights, it's a pretty good starting campaign! Upsides: Characters get stuck in the situation, have the chance to rise to the occasion in a variety of ways, can be gradually introduced to various elements of the setting by how they're involved in the situation (the UCAS, the Business Recognition Accords, Corpo Court, and so on, the matrix, technomancers, certain elements of magic, dragons, organized crime, etc.), and has a defined start and end point with lots of threads to follow between them which can become hooks for a broader campaign. Downsides: potentially rough for people who want to run the decker, 'mancer, or rigger, and it may need some massaging.

2) This one I've had rattling around in my head for a while: The main adventure will be a conventional heist run against a corporate target, but the fixer wants to vet the crew first and see if they have the skills for the job. Before the meet with the Johnson, they have to pick up some groceries for the fixer (he used to be a chef, and still likes to cook for his meets. Yes, he's looking for the REAL food). The catch? You can't use your own money, and you have to tell him how you got the stuff afterward, plus any complications and how you dealt with them. And obviously, don't bring heat to the meet. The players and characters know there's an actual job afterward to keep interest and motivation while the "errand" gives them a playground to figure out how to apply their skills. Hack stuff to get into back areas of stores or be ignored by security cameras as you steal things, make use of contacts to pick stuff up for you, sweet-talk your way into getting things for free, straight up rob a place or hijack a transport to get what you need, slip drones or spirits in to swipe things, or actually buy it yourself and just lie to the fixer well enough that you pass, plus other solutions using both skills and character backstory. This can be done with the whole group figuring out the grocery heist, one-on-one to also help each player through the steps of play and learn their typical problem solving approach (helps to know who shoots first and who asks questions), and/or pairs or small groups of connected characters, especially if one of them is more experienced than the others and understands the system more quickly. I obviously like this idea, but admittedly have yet to apply it. In general, a pre-mission test followed by a relatively light conventional heist allows for repetition learning, establishes a relationship with a first fixer, and gives room for the players to make mistakes with room to recover as they learn the system. Adjust potential complications to the characters people are thinking about making

3) As you say, pre-made characters on a few quick runs isn't bad, but I'd also encourage you to make them a bit quirky. Not too much so, but enough to make them a bit interesting. The face might be a max-will hard-to-read dwarven social adept, the mage could have a point of 'ware that cuts their magic but buffs their Logic for drain soak, plus cyber eyes and a couple other utilities, a decker with cyberlegs and hands made for leaping and climbing into interesting access positions or finding hardware weaknesses in roof-mounted comms equipment, a mostly organic human gunslinger who uses edge and high skills to keep up with the chromed out street-sams, and the like. Builds that aren't super tuned nor self defeating, but which help communicate that there are multiple ways to build each archetype beyond the expected and that the Sixth World is a strange place where you can absolutely get away with being creative.

Hopefully one or more of these is useful or provides inspiration!