The confusion here is that "expert" is both a noun and an adjective. You can be an expert in a subject, or you can be expert in that subject.
So when you say "Not all X are Y" (where X is plural) you have to be clear about what part of speech Y is.
Not all fruit is orange (adjective)
Not all fruit is an orange. (noun)
In the same way:
Not every consultant is expert (adjective)
Not every consultant is an expert (noun).
If you choose a noun, though, it must match the subject:
Not all hats are fedoras.
Not any hat is a fedora.
Note that using a modifying quantifier like every, some, all, etc. changes whether the subject is considered be singular or plural.
Not every boy is a student.
Not all of the boys are students.
More on subject-verb agreement with quantifiers