When using Radiant Greninja's "Concealed Cards" Ability, can I discard an Energy card from my hand, and then later during my turn draw 2 cards?
No, you must draw the 2 cards immediately after discarding the Energy card. "Once during your turn" means you cannot use this Radiant Greninja's Concealed Cards Ability multiple times during the same turn.
Source: Astral Radiance FAQ; TPCi Rules Team (2022-05-26)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH10_46
SWSH10_46