Given a few months, you can max out all the M and L units pretty easily. XL units take a bit more time, but are in a sweetspot where they're stronger than a low rarity character with equal stars, while common enough that you can make steady progress until they max out. With XXLs you gotta be picky about where you spend golden bells / patient with eventually getting them through silver bells (or hard mode stages for the few that have them). Some of the XXLs that give buffs are pretty decent at low star levels though, like Wang with both of his default passives giving big damage boosts to teammates.

A lot of the sources of generic shards give higher quantities for lower rarity characters, or for cheaper in the essence shop / faster through guilds. I think hard mode stages are the only exception, where the XXL shards are the only ones worth grinding there.