filozofo

Here is the point worth carrying away, and it costs you a whole family of words. Filozofo is the person and filozofio is the field, and the difference is the -i-, not a suffix you add to make somebody. That is the Greek pattern, and Esperanto keeps it wherever the international vocabulary supplied both halves: astronomo and astronomio, geografo and geografio, biologo and biologio.

So filozofisto is wrong, and it is the mistake everybody makes, because -ist- really is the normal Esperanto way to turn a field into its practitioner. It works when the language has only the field-word to start from: fiziko gives fizikisto, kemio gives kemiisto, matematiko gives matematikisto. Where the -o and -io pair already exists, the person-word is already there and you must not build a second one.

Notice which way round the two halves sit, incidentally. Filozofio is longer than filozofo, so the shorter word is the person and the longer one is the subject. That is the reverse of the intuition English gives you, and it is the whole reason the mistake is so persistent.