You know ecash wouldnt be one unified anonymity set right?

It would be a fracturing. Opposite effect of what you want.
Each mint would have it's own separate anon set.

and regarding on chain bitcoin
small dark pool >>> large transparent pool
how large a group is irrelevant for anonymity if it's users can be picked out thru KYC or weaker pseudonymity of non-coinjoiners (majority of users)