I'll do my best here; the Octo Expansion still has the odd challenges like the main single player campaign such as just heading for the goal or collecting X items to complete the stage. But they're often mixed up in a way where you have to think differently on how to tackle the stages (some force you to use only a main weapon, sub-weapon, special weapon, or no weapon at all). While other stages require you to meet certain conditions, like pop all the balloons before they float away, guide the 8 ball to the end, break all the targets within the time limit and so on. Most stages give you the option to choose one of the weapons (or weapon sets) to tackle the stage with, with the tougher options awarding you with more credits than others. The objectives sound simple enough, but most of the levels are significantly more challenging than in the normal campaign; do not underestimate them, especially the rematches with the bosses. The game's climax where you want to finish the story mode is very challenging and a lot longer than I expected with a great boss fight. It's a significantly more rewarding and better finishing payoff than the main single player campaign of Splatoon 2, which while nice felt like a step back, particularly with the final boss.
You don't have to complete all 80 stages but you do get rewarded with gear to use in multiplayer for each train line you complete, as well as an extra weapon (albeit a reskin) for finishing the story.
I'd recommend the expansion as it really does put you to the test and challenge you on speed, timing and accuracy a lot more than before.