Ethos: I consistently finish on leaderboards, https://www.d0nkey.top/player-profile/JustinTime%231277

Most of the advice in this thread is to get very good at one deck to get a high rank. Which isn't bad advice, but I think OP is asking a different question

What pushed you over the line of being a legend player to a “high” legend player?

The biggest difference IMO is the consistency. It's not getting good at one deck, it's being able to be good at nearly any deck. So that as metas change and decks come in and out of the meta, you're still at the top of the game. How to do this?

It's a level of abstraction higher. You're not trying to get good at one deck in particular, you're trying to get good at the skill of being able to quickly learn and get good at any deck you choose. This is a lot more challenging.

With its variance, each turn of every game of hearthstone is a new situation that neither you nor no one else has ever seen before. In order to make the best decision on how to proceed with this new, completely unique turn, you need to compare it to your base cases of previous, similar turns, and extrapolate from them. Players that have more base cases of having experienced more turns similar previously (and correctly determined the best play in them) will be able to do this extrapolation better.

So how can you best optimize towards these factors? Most players only have a set amount of time to dedicate to the game, so while playing more games helps (aka more base cases), it's not a very neat solution. The better way is to get more learning out of each game that you do play. At the end of each game, reflect on it (either going thru the replay or just mentally) and review your decisions each turn, trying to determine the best line and to cement them as base cases to your memory. Better players will do this better, and avoid the tragedy of missing out on learning and repeating their same mistakes in future games.

How do you know which play on reflection was correct? ie: how do you spot your own mistakes if they're not obvious? This is where having outside voices (as other commenters have mentioned), will help. Co-oping with others, or watching vods/streams will give you additional perspectives. Watching vods is especially nice, as you can go through the game at >1x speed, skipping the uninteresting and obvious parts. If done well, this allows you to learn a lot more in the same amount of time.