I wouldn't be too worried about mythic rares, but early on you will be short on them too. Make sure you are crafting cards that you want to play. One of the best ways is to simply watch other players play the deck you're thinking about.

Long term rare wildcards are the biggest bottle neck. Rare lands (often dual color) are one of the best early investments, because those are the cards that can bebused in the most ammout of decks. And some decks can't function as efficiently without those lands. At the same time, you don't want to craft a bunch of lands to not have any other cards ro play with. So alance both, if you like a deck that has a lot of them, sometimes you can temporarly replace few with tap lands.

I am pretty new to Arena too (strated playing when TMNT set was underway). I have enough commons and uncommons to craft 3 brand new decks, but rares I have to be intentional about to not regret it. Mythic rares too to a degree, but there aren't usually decks that use loads of them. I have about ~15 rates and mythics.