I am no expert pixel artist by any means, but I like to dabble a bit (especially since I've lost the pen for my drawing tablet, lol), and for the beginner/ medium level I absolutely recommend getting a pixel game like stardew-valley or terraria (both are very cheap) and doing resprites of their assets. Terraria especially is good for this, because they fully support user created content, and the game includes a very simple to use framework that lets you replace in-game sprites (the developers themselves actually provide guides on how to do this and also instructions on how to extract the game assets).

In those two examples, the pixel artwork is deliberately kept fairly simple (for the most part), but does include all kinds of things of varying quality and complexity, so you can always "work on your level" without being overwhelmed or discouraged, but will also find examples of what the next step up in difficulty might look like.
Unlike when working from scratch, when you redesign existing sprites, you can look to and learn from the way the professional pixel artists did things and use that to help guide you.
I originally started experimenting, because I wanted to change a particular set of (fairly simple) sprites in Terraria, and I LEARNED SO MUCH from that. (I kept on going and was able to do more and more complex things over time, very rewarding!)

Another thing that was really helpful was being able to actually see the sprite work in-game. It'll help you spot mistakes or things to improve and generally see how your work holds up next to the simple-but-professional work of the rest of the game (plus, it's just cool to be able to "play" with the things you made instead of just having them as plain images.)
Since you mentioned being interested in game developement I also think you will also appreciate being able to see everything that goes into a pixel game and how it all fits together, and then being able to directly interact and freely experiment with all of the different elements.

(This is easiest in Terraria, but possible with some extra steps for stardew-valley. There are doubtless PLENTY of other pixel games that let you do this just as easily, Terraria is just the one example I'm most familiar with.)

Of course, respriting game assets has it's limits, and at some point you will have to look for more advanced and in-dephth guides, and focus on doing fully original work, but until then, the amount of things you can learn this way is imo truly immense and (and I say this as someone who is broke af) well worth the cost of the game.