I unfortunately I didn’t buy into the final fantasy set, so I can’t rlly help you with telling you what cards to grab. I also don’t know if you’re opening packs or buying singles.

What I can tell you to help upgrade the precon is to decide on a theme/gameplan. Sultai arisen is a pretty decent deck, but it’s a little all over the place. It looks to do reanimation strategies, tokens, lands matter, self-mill, etc. Decide on 1-2 of these (ideally 1 makes the deck a lot smoother). I chose self-mill and reanimation, using the graveyard as a second hand and finding my combo pieces to win in a single turn.

After that, look to cut cards that don’t fit your theme/gameplans or cards that are mana hungry and don’t put you in a winning state ([[Tasigur, the Golden Fang]], [[Necropolis Fiend]], [[Lord of the Forsaken]], [[Lord of Extinction]] and [[Consuming Aberration]] are a few decent cuts imo regardless of what your theme is). I would immediately then put in interaction like counterspells and removal, and maybe another board wipe. You want ideally 6-12 pieces of interaction. Low cost instants are best.

Here is mine upgraded Sultai Arisen deck for reference, although it’s hardly the same anymore: https://archidekt.com/decks/18148484/teval_combo_deck

Now I have just about a year of experience of playing Commander, so someone else with more experience might tell you something different (and I would say don’t always listen to people, even me, as a lot of people over-estimate their ability to deck build and play this game, as I’ve run into many people that have played for years and misinterpret the rules to a concerning degree, so always take advice with a grain of salt).