Off the top of my head, I’d say the best thing you can do is just backpack as much as possible and find out what type of trips bring you the most joy and prioritize those. You’ll quickly recognize the skills that you’ll need to develop or improve to enjoy those trips and through practice you’ll get “better”.
So I’d say start with a bit of reflection on what you’re trying to improve at and why.
Do you feel tired and physically worn-out at the end of a day of hiking and know you could be in better shape and thus enjoy it more? Then work on fitness.
Feel tired at the end of the day but in a good way and feel like you just didn’t see enough and want to hike more miles? Then I guess plan harder trips and focus on fitness, lightening you pack so you don’t feel worn out if you hike 25% further.
Like the solitude and scenery but find most places you’re backpacking are too-crowded for your taste? Work on trip planning and cross-country routes, where you might lose some of the crowds.
Tired of eating the same meals over and over? Focus on recipes and meal planning.
Wanting to do harder peaks that aren’t just walk-ups and scrambles? Read up on mountaineering/climbing and get involved with groups to help you build those skills.
I’m sure you get the picture by now : )
To be honest, I feel like once you reach the intermediate level of backpacking you’re in a great place. Backpacking can have a steep learning curve, but it ends up at beautiful and rewarding plateau that you can hike on forever.
Once I developed a solid foundation of backpacking skills, I basically just pivoted to focusing on figuring out how to do the trips I most enjoyed as often as possible. For better or for worse, I haven’t focused on “skills” in general in years . . . I’ve certainly learned new skills that have helped me with enjoying trips in different conditions, but I learned those skills only because I needed them for the trip. For some of the trips that I didn’t enjoy that much and likely wouldn’t repeat, I don’t find any need to try and learn related skills or keep those sharp.
Once you learn the skills to be safe and to have as minimal impact as possible, just focus on maximizing comfort and enjoyment. For some people that will be developing a fitness routine to do 25-mile days easily and learning how to pitch a 5′ by 7′ tarp to provide adequate storm coverage, for others it will be figuring out which wine to pack in that will pair best with the filet mignon they’re grilling on the first night.