"pensi" is far more general.
"taxi" is only to estimate some value or quantity. "pensi" is generally to think.
Why " Sofia taksas sin tre bela " and not " Sofia taksas mem tre bela "
"Mem" is used for emphasis but "sin" is used for the reflexive. In English, we use the same words for both situations, but not so in Esperanto. So, in cases where "mem" is used, it can be removed without being grammatically incorrect (e.g., "Sofia mem faris la laboron" = "She herself did the work").
How does one say, "Sofia considers her very beautiful." when "her" is someone other than Sofia?
Sin is a gender neutral pronoun which references the subject of the sentence when that subject is in the third person.
"You used the wrong word. • Sofia rates herself very pretty. • Sofia considers herself very beautiful."
So... why "Sofia consider herself very pretty" does not work? English is not my first language.
"Consider" is the wrong conjugation. "Sofia (she) considers", with an 's' would work.