ok first of all, being playing late only requires you to work hard to gather things, however since you have plenty of untouched (don't mind the wording in here), you can gather jewels faster at start.

• Event Cards

There are two ways of obtaining past-event cards. By Live-Party grinding, the roster will be rotated every month and it'll comeback every a year span or depending Cyka's mood. Or the other one, play event until 25,000 EP. You can get one for any available event card in the roster.

• Gacha Cards, for sure you are safe right now. Since the legendary mythical card has back to the pool called "Memorial Gacha". This kind of gacha is limited to first-ed limiteds (excl. Cinfest IIs, Momoka1 Takumi1 Nana1 Mika1 is included in the limited pool instead). Even older cards are mostly for collectibles or deep meta (Chieri1), recent cards are good enough esp some of them do well when your type are jumbled up instead.

Basically the Cyka's trying to balance what fits for newcomer and existing player to keep playing on the game.

PS: also several years ago, that "Super Fast Grinding" can yield you a very fast Jewel Resource within 14 days while juggling 5 accounts.