I largely agree with the other feedback, but I wanted to add that it feels unnecessary to require that the player collects all the coins to advance a level. Partly because it's not clear up front, but also partly because it feels overly difficult to go back through a level if you missed a coin. This is a really strong project overall and impressive for a first project! And there are a few really fun details, like the "funnel" in level 3 and the "glass" covering the falling portion of level 3. Really creative and versatile level designs! I also had some slow loading times - you may want to check out the Unity Profiler to see what's going on (especially in level 4).
But in the end I find it too frustrating. A simple thing you could do is instead of requiring all the coins to award a bonus for collecting them instead. Why else is it frustrating? Because it is *very* hard to see where you'll land. One way to address this would be to create a "fake" shadow directly underneath the player to aid in landing after longer jumps. I would also think about what kind of challenge you're trying to give the player. I think "punishingly difficult" is a genre, but when it is done well it usually is a quite "scaffolded" experience that only throws one new thing at the player at a time and gives lots of time to practice. Your levels do this sometimes, but other times it feels like a bunch of new things all at once without any ability to really practice them. The question I'd think about is "How do you want the player to experience challenge?"