When designing a run in SR, I tend to think of it more as designing a puzzle or obstacle course than as an equation where you are trying to make the two sides roughly equal.
When runners are hired for a run, it is with the expectation that they are competent to the job. That barring surprises or accidents, they should be able to do it well enough (it is why you are hiring professionals, or at least professional criminals). A shadowrun team almost never wants to match their strong area against the targets strong area, they want to put their strengths against a targets weak areas, and when they manage to do that, they should succeed fairly easily (sometimes I won't even roll dice for those parts, unless I feel they need a chance to show off how good they are, or I want to build tension or hide when the real challenge is coming). And when things go wrong, obstacles can pile up quickly (high threat response teams are called, astral mages with spirits appear on site, and so on), so then their challenge is how to get out quickly before the obstacles become insurmountable.
You can't really control how they will choose to overcome the obstacles that you put in their way. So for example, you could make it easy enough for them to sneak in to their target -- say the facility is new and people don't know each other yet so it is pretty easy to imitate employees and walk in. But the lab has enough people in it that an alarm is going to be sounded almost for sure, and the guards may not be well coordinated but they know that their job is to delay the runners until HTR shows up.
The runners may choose to shoot their way out as you expect, but they may come up with a pretty brilliant plan to exit through a third floor window and slip away invisibly. If it is the latter, I'd let them mostly do it -- they used their skills and equipment and smarts to get around the obstacle, good on them!
If they do shoot their way out, the obstacle is "getting away before the high threat response team arrives", not "win a shoot out with basic guards". Focus on the blaring alarms and likely imminent arrival of heavy hitters, and have some puzzles for them to solve. Two guards are using a corner in the corridor for cover, does the team rush them? sharp shoot them? Use clever magic or hacking? How long did that take? Then they get to an internal door that was open before but is now locked, how are they getting through? (or do they find another route?). They need to get down a staircase with guards at the bottom, how do they handle that? As they approach the exit a lieutenant and a few more guards have prepared themselves to try and stop them from exiting.
After each step along the way, you tune the next obstacle easier or harder (or weirder) depending on how the last one went. Up the professionalism level or add another grunt or two, or give this set better weapons. They used spirits on the first set of guards? Then an astral mage is in the staircase and will try to dispell their spirits (or has spirits of their own). They are romping through? The opposition turns off the lights and throws tear gas, are the runners all ready for that? And so on.
ETA: specifically about point 3, sometimes I use the goons from the books (or from an adventure) but for 'just goons' (gangers, basic security guards) I just use dice pools. Something like "gangers: 8 dice with their cheap light pistol, 8 dice dodge, 8+1d6 initiative, 12 dice soak" and generally that is all that is needed. Higher level opposition I'll sketch out in a little bit more detail to make them distinct, maybe one has a cyber-arm, another is a troll (increase that soak pool!), and a third is a weak adept with an extra 4 dice in pistols.