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Basically, a good way to never trust "it's okay, the data is anonymized" again is simply knowing what the "Hemisphere Program" is.
https://www.eff.org/cases/hemisphere
In short, the US government got access to number from, number to, datetime, length and sometimes location information for every call passing through AT&T's network from 1987 to today.
Then they ran an algorithm to de-anonymize every burner phone based on behavior. They did this because maybe some of those burners were used by drug dealers.
While Signal does have problems it is still pretty solid. You are right about the metadata though.
Anyway Simplex Chat exists and if you are really concerned you can use Briar