This is more general, but I think improving at Dominion mostly follows these steps, not necessarily in this order.
1. Learning not to underestimate the basic Treasure cards. They're good. They're pretty much always a good buy until you get to ~lvl 20, where some games start to turn them into not as good buys, Silver in particular. This also includes learning how many actions you can fit in a deck. If your action doesn't give +1 or +2 Action, it's going to make missing an action play happen more often. And this should be obvious, but if your action draws cards, the chances of you drawing cards you can't play skyrockets. This is why people are recommending you only get 2 Witches and skip Smithy, even though Smithy is a great card. If you get all of those terminal +Cards, you're not going to play them enough for it to be worth it.
2. Learning which actions are generally worth buying and which ones are not. This comes with practice/playing the game enough and noticing what people who win tend to do. From this game, I don't see any big issues.
3. As an extension of 2, learning which actions fundamentally change the game. One example being Witch: giving out Curses is a drastic change. Once again, no major issues.
Starting from here is the growth that begins to happen concurrently.
4. Learning not to overestimate money. In this game you want all the money you can get. But sometimes, not so much. If there's a strong combination of actions out, money makes hitting your action chain more difficult. But you do need to buy some money to buy the action in the first place, and so on.
5. Learning general rules of thumb. For instance, "In a game where Cursers exist, you want to get 2 of those Cursers." I think this is the longest step, and with new expansions coming out it takes longer and longer. This isn't a step so much as a thing you constantly learn. Rules of thumb almost always comes from experience, either by experimenting or by getting utterly destroyed by someone 15 levels above you.
6. Learning when to break the rules of thumb. Definitely the hardest step to learn. Opening Silver/Silver on this board with Militia isn't something you'd normally see. But with Witch taking up the terminal actions slots your deck can support, skipping Militia could be worth it.
7. Deck control. shark_bait has a nice article on this. Essentially, if your deck has 3 cards left, you have a Smithy in hand, and you have not played a Witch yet, skip the Smithy because your deck has a Witch in it, and it's more important to play the Witch and hand out curses.