That's why I think you should start with the target's organization and resources, and likewise the rivals'. That stuff will always be useful and if you know what it is ahead of time you can just move the stuff around in response to what players do.
Yeah, this sounds good. I'll have to read the node thingy. For turns, I have done well with just using time before. (I should get around to telling the story of the Security Officer the PCs needed to get information from, preferably quietly). It's better to cut back on making detailed content than content altogether, and knowing what events will happen is more useful than stats, in my experience.
I'm not too worried about stuff ending up unused; I'll always need another opponent or a description of the interior of a building. The best part of a dumpster full of unused ideas is mining that dumpster later. I think I've only had 2 out of 12 missions where they avoided the stuff I've prepped almost entirely, and that was because I prepped very little of what will happen and where/when. I just need to have stuff for them to interact with no matter which direction they come from.
On stuff that isn't related to the target and whatever security there is (rambly train of thought): For the other groups/people I was thinking about a theme/MO for their group, how many there are, how good they are, what they're good/bad at, and other groups they have more of a friendly competition thing with or have a "kill on sight" thing going with. Probably a "team" of 1 with good skills, but who can be pretty easily overcome with numbers/force, and a group the size of or 1 larger than the players' group who are idiots and will probably end up drawing too much attention if they survive long and already have a lot of enemies. Both of those present potentially interesting challenges to the players, and which the players would benefit most from knowing about ahead of time. Also, the movie used both (somewhat badly), which means I at least qualify to write a mediocre action movie (which is about the level I'm shooting for). That, or we have cliches for a reason. The same reason a tall building means a helicopter will show up, which I predicted in the movie, even though it didn't matter to the movie.
Allowing the players to learn about some but not all of the groups in advance if they put their prep time into that would be interesting. It means they can try to learn enough that they can set them against each other, or prepare for their strengths weaknesses. That also means that the opponents have to be able to be dealt with without knowing about them ahead of time, because they can't know about everyone. It sets good limits for me. I should add "how can the players find out about this group ahead of time" to that list in the previous paragraph.
I also want the groups to interact with each other, even if they PCs aren't present or otherwise encouraging it. Setting up checkpoints/funnels that force the groups together would work, if I know when the groups will hit each checkpoint. Scheduling when each group will arrive, which direction they'll go, what stops they'll make (if any), and about how long it will take them to reach the goal if nobody interferes should be enough. "Arrive at 8, go in the front door, get by the front desk and to the secure elevator at 8:30, reach the final security zone at 8:35" is plenty, especially if I know when the elevators will be shut down, how long any extra delay at the front desk will take, and what other groups will be in those spots at what times.
Whatever the facility is, will matter a lot. The number of groups and their variety of MOs means the players should interact with at least one beyond the guards and target, especially with funnels/checkpoints. So making sure something happens to the groups who are there will keep things interesting, and the players will end up walking into one of the pre-scripted events. Continuing the example from before, knowing the elevators will be shut down at 8:40 means the group that talked the front desk into letting them use the security elevators as a shortcut will get screwed if there is a delay, and have to take the stairs and possibly run into other groups. Again, I want quantity, not quality because I'm expecting 75% to not get noticed by the players. Stuff like car wrecks stopping traffic, hotel guests setting off a smoke detector making popcorn, and security taking an unexpected break because they should *not* have eaten that last night are easy to write down, and moderately interesting for the players to come across.
Knowing how competent and how many the target's security (I'm assuming there are separate facility and target's security groups, so they can interact) is good for dealing with the PCs not showing up first. If someone else gets there first, the target's security should have reduced forces, but be on higher alert. Separating the 2 security forces also means one can be infiltrated without the other being bypassed completely.