Been skating for over 40 years, and what improved my skating the most depends on what decade it was.
As a kid / teenager, is was the amount of time I spent on the board.
In my late teens / early 20s it was finally having grown into my adult body and learning to use the strength that came with it.
In my 20s / 30s it was working as a skateboard / snowboard instructor. Learning to break down trick movements in a way I could explain to a 5 year old, and being able to watch someone and understand what they needed to adjust made me so much better at analyzing my own skating.
Then at 40 I destroyed my knee and had to learn to walk again.
I wasn't sure if I'd ever be able to skate again, and it was a very slow process. Now at 50, I know what my knee can and can't do, and my sessions are all about how much fun and exercise can I get before my body gives up and I have to go home.
One thing I started doing about five years ago, is going back to the absolute basics, and then seeing if I can do those things all directions. Front side, back side, regular, and switch.
A switch front side kick turn on a ramp is as scary and challenging to me as jumping down any 10 stair rail when I was 20. Same adrenaline spike, same sense of achievement, and the danger level is almost 0.
I still to this day can not push switch. Every session for the past couple months I spend a few minutes trying, and bloody hell do I feel awkward.
So to finally answer your question, the thing that improved my skating the most is spending time focusing on switch riding and movements. I wish I'd done it 30 years ago.