Not sure if this is helpful to anyone, but how I play this game sounds different to how everyone else does, so I thought it was worth sharing. Also bear in mind I have spent almost zero hours making course so far, so these ratios will change at some point in the future.
I spend about 20% of the time on Endless mode. I have an actual endless run going on Easy and Normal (I'm about 80 courses deep on those with more lives than I'll ever lose), and then I flick between Expert and Super Expert for more interesting challenges. I've found that Super Expert can often be brilliant for finding great courses, and I'll skip those if they'll ruin my Endless run and return to them later to finish.
I spend about 10% of the time on Story mode, because I want to progress the story, and it's a great way to learn new mechanics and course ideas and the creativity there is spectacular.
I also spend about 10% of the time playing courses from here, and from other friends, and those are the World Records I chase most often.
But the rest of the time I load up a bunch of people I watch on YouTube who I love watching. People like CarlSagan42 and Grand PooBear (and I am more than well aware that if you're not conditioned to these people in advance you'll hate their very cores as soon as you load a video). These guys are some of the best players in the world and they have people around them curating their level selections for them. Don't bother live streaming them. The YouTube content is where you'll get the best of what they've been doing on their Twitch profiles. What I'll do is load up one of their videos, which are often categorised so you can decide if you want troll courses, courses which are basic but have great ideas, glitch courses, or super hard courses, for example, and then, when the creator ID appears on screen I pause the video, play the course myself, and once I've finished I'll watch them play it. It works for me because the courses are always of a much higher quality than whatever random shit you'll find on the Internet, you'll learn a shit load because they'll force you to learn weird tech or new tricks, you'll improve your skills over time, and then after you're done you get to watch someone much better than you spend time learning the course the same way you did, fucking up in the same places, doing things slightly differently (better!?), and having a similar experience.
For example, here is the section from Carl's video that I watched last night before I posted that Tweet. The ID comes up, I pause the video, I spend fucking ages beating the course, then I watch Carl beat it himself. After I was done I realised from Carl's stream that I did the bit with the side Thwomp and the conveyor belt wrong because I was supposed to go in front of the Thwomp, which would have made my life a lot easier, but that's all part of the fun, and I love the "reward" that comes with beating a hard course of watching someone else suffer the same fates.
Dunno, maybe I'm just weird as hell and no one else does this. Perhaps it also helps that I'm the kind of person that will sit down for three hours with a single course making microscopic improvements until I can beat something, but my favourite thing is loading up a new stream knowing that I'll have five or six great new courses to get into that'll take up the next few hours, and I'll probably improve as a player by the end.
No idea if that's helpful to anyone.