I echo what kirbysdl and FANAttIC said - practice makes perfect. Specifically practice with mulligans and sideboarding is very important for Merfolk. We don't have cantrips and deck manipulation spells other than
Silvergill Adept, so our open hand is what pretty much determines the course of the game. So understanding when to keep and when to mulligan is important. Understanding what to bring from sideboard and what to take from the deck is also critical as
Abrupt Decay and other anti-Chalice cards are common responses. This really forces you to acquire a good understanding of different decks and how Merfolk interact with them.
And speaking about big events, depending on your pairings, Merfolk actually has good chances to do well. Miracles is still the dominant deck with about 30% of the big tournament metagame and Merfolk prays on Miracles. On the other hand, it is much harder to win over Reanimator as it is so damn fast when their hand is good and
Elesh Norn, Grand Cenobite is a complete devastation of the board.
I'm not trying to sell you Merfolk, however this deck is relatively cheap and many cards from the deck are transferable so definitely Merfolk is a good choice for someone who wants to start playing Legacy. Taking into account that you already have Burn, at some point you will be able to assemble UR Delver-type deck (if you decided to get fetches and duals).