This is all just spitballing my opinions out loud - take anything useful and leave anything that doesn't seem useful - no gospel here =)
Creating a campaign means rolling up characters that you and players intend to live with hopefully to retirement age.
You can't just "unleash the kraken" on them and that to create the air of spookiness you need to play on the suspense and threat of the oblivious characters blundering into the horrible threat the PLAYERS are sure is there.
Every adventure is basically a haunted house walk where you are not sure where and when the jump scare comes.
So what is everyone doing in the meantime? Getting on with paying the bills and generally living and inter-relating.
A campaign rather than cinematic one-shot is going to focus on the characters, their problems, and their relationships.
The problems are your key in: every character rolled has a backstory or history that you can mind for problems for them to solve now or in the future.
Putting those problems in front of the players, usually strong enough to make them unavoidable that they take action to resolve them, is your adventure hook at each stage of the campaign.
The "main excuse to adventure" will depends on the campaign theme and style you intend - space truckers are always moving on to finish the next contract, marines are getting their next mission, execs have the next project on their annual review, etc.
I'd start by putting myself in the character's boots, and figure out missions based on their day jobs, and what sort of trouble they might get into as a side hustle.
And consider who are buddies and rivals and be sure to work that in... rivals will be scheming to usurp or undo each other making for a web of low-level tension that also provides mission/adventure hooks.
Once you have that outlined ... then comes the fun part: layout out tidbits and clues that something, somewhere is horribly wrong and how to build up clues and rumors to unveiling it.
This is the cinematic one-shot style adventure where you will threaten the character's lives and let the xeno strut ITS stuff.
The longer you can hold off between session 1 and that "now, unleash the kraken" - the more time you have to get the players invested in their characters and set them up for the horror.
The PLAYERS know its inevitable that they will face xenos - but everything else is still yours to play with - how, when, where, why, what xenos, etc.
If you want to see an example of this in action... just watch ALIENS Special Edition and check out how long it is from start of movie to the first encounter with xenos in the Atmosphere Processor.
WE all know its coming... but how about the marines?
What clues did the marines get?
Ripley's report no one believed.
Some stasis facehuggers.
The resin hive - where Burke says "busy little creatures" - everyone except Ripley is still thinking things are the size that can be "attached to his face"
And then it all breaks loose.
How can you have fun with xenos without a total party kill in a campaign?
Along the way introduce piles of NPCs... then let the xenos go on a slaughter through those after the players have built some connections with them.
So to build the tension - have adventures about everything else in the alien universe except the xenos - there's tons of stuff in the book to mine from.
Political and corporate intrigues going on with the characters caught in the middle.
But keep finding more and more worrisome clues that the darkness of space is coming closer.
Spaceport bar rumors of colonies gone silent with corporate and government coverup - starting with LV426.
Rumors of ships gone lost (like the Cronos).
One of the characters happens across Morse's book.
There are hooks peppered liberally though the core book - they could probably be woven into the sort of suspense you want.
There's a couple of fun opportunities in the book too: one is to run into Paul Van Leuwen in his hunt for xeno / corporate conspiracy.
Another is to run into Michael Bishop in his hunt for xenos.
Why not consider your high-level campaign overlay be the race/conflict between Bishop and Van Leuwen, and end up with the PCs getting caught up in that after a few adventures?
That news / rumor / conflict can be hinted from adventure 1 as well using news and NPC dropped hints like grumbling between dock workers on big wigs both trying to get their ships launched first, etc.
So now to your question - what should be in the opening adventure?
Let the players tell you through their characters!
It's your character's origin stories as an adventuring group.
During session zero - the group rolls up characters and helps each other establish how and why they are a group now.
At the end of the session zero - take 2 steps: outline the public problems facing the group - paying for living etc and ask their characters plans to resolve those problems.
All of that is your mine-able fodder for the "problems" that setup the first adventure.
That gives you some initial intentions to build from...
It's your job to help get the players to the end of their story.
It's the players job to tell you how the story starts and set the initial direction it takes based on their hopes goals and intentions for their characters.
We live, as we dream -- alone. ~ Joseph Conrad