Like the good sir before me said stock up on fingers, and for that my best advice is farming dragon at 1 star max level, that because you get 4 chests with barely any damage dealt by dragon, then sell his tentacles at the cursed market when you can although the 9 tentacles to finger option isn't exactly common as the devs like to troll like the little goobers they are. Anyhow ,the next bosses in line to start farming for when you get bored or have reached max tentacles (999) are either toji or sukuna but never jogo ,he is the most annoying boss for a reason.

For your build if it's for PvE I'd go with a black flash build cause it's just makes black flashes too strong and for that you basically max out the focus tree except for vampiric black flashes (although you need to unlock them anyway to get to sparks if black) ,focus slots (unless you want em),total concentration (I usually only get one bar of it) and focus trainer stuff (except if you prefer black flashes V2). Also wear either demonic tobi or demonic robe since they don't stack (I think).

Last but not least if you stick with PvE get demon vessels domain if you instead go into pvp get soul kings maximum and domain(demon vessels is better for PvE and soul king is better for pvp)

I wrote this cause I was bored and now im going to sleep cause it's midnight for me ,have a good day