Which pity path should I prioritize?
Finish the standard Mythic pity for Gojo first because it guarantees at least one premier carry for all content.
Once your dailies and events cover 2k+ gems per cycle, start banking for Sukuna's limited Secret pity so you don't stall your entire account.
Do I need Sukuna if I only play casually?
Not immediately. Gojo plus solid supports clears story chapters, raids, and most events. Sukuna becomes mandatory only if you plan to afk Endless ladders or compete for top raid times.
What supports work for both?
Sprintwagon, Song Jinwu, and Alligator all pair well. They provide the attack, economy, and mitigation that let Gojo spike harder and give Sukuna the time he needs to stack curses.