Elite is exceptionally complex - there are tons of controls for just about everything.
Flying a space craft, well, it feels like you're flying something, from take off to landing.
Once you come to terms with all the controls, and get the hang of take off, fly and land... that's about 80% of the game.
Learning to do these things is not overtly difficult, but the learning curve is a bit high, and that's fine.
After that, the remaining 20% falls into two particular areas:
12% Career-related - What is it you want to do? Are you a freight-hauler? A trader? A mercenary? A Bounty Hunter? A deep-space Explorer? A Xenobiologist? An Exo-Archaeologist? A pirate? A corporate raider? A miner? A little bit of everything? Figuring out what you enjoy doing usually doesn't take all that long. Learning to be good at it... that's not a learning curve, it's something of a vertical wall. Unless someone is actively showing you, figuring out how to get the best trade deals, where to find the best rocks to mine, which way to go to find places no one has seen before... you're very on your own. The game offers little to guide you and throws a lot of resistance in your path. This is likely the single most difficult aspect of Elite. Combine this with the fact that Elite is far from "complete" - that every single profession so needs some meat on its bones, and you get to where we are right now - not-a-season-3 Beyond - which is working to do just that, flesh out the skeletons of careers we currently have. And this will mean re-learning how to perform these careers to the best of your ability.
8% Is learning all the little details - Signal Sources, Materials, Data, where to find these things, how to find and collect these things, and so on. Generally this is not all that difficult, but there are volumes of information to remember.
Fighting Thargoids in unengineered medium ships is still challenging. The rest of it is sort of relative to my experience and wallet. At this point, there's not really any other challenge left that i haven't progressed past. On the one hand, I do wish there was something to challenge what I have access to, but on the other hand I do feel like I've quite earned the position I am in.
Your wallet? Unless I missed something somewhere, there's nothing you can purchase that gives you any advantage over anyone else. They designed it this way on purpose. This isn't SC after all.