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.

I'll say this, the expansion is a much greater challenge than the Splatoon and Splatoon 2 single players. I have been chipping away at it a little everyday. I have collected all 4 things. I do plan to finish all missions.

the Octo expansion consists of 80 non-linear missions. these missions are challenges that are focused around one to three specific weapon that remix all the mechanics of the first campaign as well as a handful of new mechanics. in some challenges you have to navigate a giant 8-ball through the level being careful not to knock it off the edge while still avoiding enemies. in other challenges you can have a limited ink tank and have to make it through the level without running out. some levels dont even give you a weapon at all and you are challenged with either surviving a set period of time or stealthing your way to the end. there is so much more variety here, making for a much better experience that the normal campaign. not to mention that these challenges are a lot harder but they never feel particularly unfair, making each one satisfying to complete. and i wont get into spoilers but this game delivers some amazing world building and lore to the Splatoon universe.

IMO, the Octo Expansion is the best thing to come out of the Splatoon IP so far. and due to the challenges being focused around specific weapons, i believe that if you are mostly just into the game for the multiplayer, then the Octo Expansion will make you a better player. they force you to learn the ins and outs of each weapon and use each weapon to their highest extent.

I thought the campaign's of Splatoon 1 and 2 were only decent, but man the expansion is right up there with Doom 2016 and Titanfall 2 for the best single player shooter this gen.

Let me put it this way:

This is the Single Player Campaign we should have gotten.
Not to say the one we got at launch was bad. It was ok, but not great.
This was great.

I guess I should have been a little clearer in my OP when I said I didn't get through the main campaign. It wasn't because it was too hard, I just lost interest. Anyways, decided to get this and I'm enjoying it quite a bit. The levels are far more engaging to me than previous Splatoon single player. Thanks for the responses everyone.

It's basically Kaizo Splatoon.