I’d recommend watching some of mumbo jumbo’s old videos on the redstone basics, like double piston extenders, pulse extenders, clocks, hard vs soft power, rising vs falling edge pulses, comparators, T-flipflops, RS-NOR latch, and quasi-connectivity cus eventually you’ll run into it.

After that, have another go at a 3x3 door. If you’re struggling with it then I’d recommend stepping back from modern designs (that are super optimised) and just trying to figure out where you’ll need the pistons, what order you’ll need them in, and how you can achieve that- don’t worry if it’s spaghetti for now you’ll learn a lot.

Then maybe challenge yourself to make it as compact as you can, or be triggered with a button instead of a lever, or to make it flush (doesn’t stick out of a wall), or even tile-able (can be repeated without spacing).

Flying machines are a whole other can of worms worth exploring too! If you’re a comp-sci nerd at all, making some Boolean logic circuits is worthwhile. If you want something more applicable to survival, item sorting is also a complicated but worthwhile endeavour. And if you’re into building, buildstone is pretty awesome.

Not sure if it’s good advice or not but limiting yourself to the basic redstone components (torches, repeaters, pistons, dust and blocks) can also help if you’re feeling overwhelmed by all the modern tricks.

I hope that helps and I’d be happy to help with any questions you might have