One who keeps the mitzvos of the torah but believes that the torah was invented by human beings would fall into the category that you mention. You can see in the answers to Belief in midrashim that not all midrashim are meant to be completely literal. However, one must be careful not to go overboard on the matter.
There is a statement that I recall (paraphrase from memory)
One who believes that all midrashim are literal is a fool. One who
believes that all midrashim must be allegorical is an apikorus.
Rabbi Hershel Schachter was once asked how one should relate to the more difficult-to-believe stories in the Midrash. He responded with a Mashal. Imagine that archaeologists one thousand years from now find an old newspaper from our generation. The headline reads “Yanks Bomb the Red Sox”.
Or consider the pre World War I headline "The double headed eagle stretches its wings to cover all of Europe"
These things are perfectly understandable to the people of that time.
Consider the statement "with one drop of ink millions of people were killed".
One must be careful to maintain the mesora in all circumstances and to understand which medrashim are "literal" and which are "allegorical" as well as which use an understood idiom.
Who is an APIKOROS?
Sanhedrin 99b–100a Different Amoraim apply the term variously to: one
who insults a scholar;one who insults his neighbor in the presence of
a scholar;one who acts impudently toward the Torah;one who gibes and
says "what use are the rabbis to us, they study for their own
benefit;" or "what use are the rabbis since they never permitted us
the raven nor forbade us the dove" (i.e., who cannot go beyond the
dictates of the Torah);
Maimonides gives a more precise theological definition of the word.
Distinguishing the Apikoros from the sectarian (min), the disbeliever,
and the apostate, he defines him as one who either denies prophecy,
and therefore the possibility of communion between God and man, or
denies divine revelation ("who denies the prophecy of Moses"), or who
says that God has no knowledge of the deeds of man Maim., Yad,
Teshuvah 3:8.
Later authorities extended the meaning even further to include all
those who refuse obedience to the rabbis, even "the authority of a
religious work, great or small" (Moses Ḥagiz, Leket ha-Kemaḥ YD 103a).
Maim., Yad, Teshuvah 3:8.
Halacha 8
Three individuals are described as Epicursim:
a) one who denies the existence of prophecy and maintains that there
is no knowledge communicated from God to the hearts of men;
b) one who disputes the prophecy of Moses, our teacher;'
c) one who maintains that the Creator is not aware of the deeds of
men.
Each of these three individuals is an Epicurus.
There are three individuals who are considered as one "who denies the
Torah":
a) one who says Torah, even one verse or one word, is not from God. If
he says: "Moses made these statements independently," he is denying
the Torah.
b) one who denies the Torah's interpretation, the oral law, or
disputes [the authority of] its spokesmen as did Tzadok and Beitus.
c) one who says that though the Torah came from God, the Creator has
replaced one mitzvah with another one and nullified the original
Torah, like the Arabs [and the Christians].
Each of these three individuals is considered as one who denies the
Torah.
Halacha 9
Among Israel, there are two categories of apostates: an apostate in
regard to a single mitzvah and an apostate in regard to the entire
Torah.
An apostate in regard to a single mitzvah is someone who has made a
practice of willfully committing a particular sin [to the point where]
he is accustomed to committing it and his deeds are public knowledge.
[This applies] even though [the sin] is one of the minor ones. For
example, someone who has made a practice of constantly wearing
sha'atnez or cutting off his sideburns so that it appears that, in
regard to him, it is as if this mitzvah has been nullified entirely.
Such a person is considered an apostate in regard to that matter. This
applies [only] if he [commits the sin] with the intent of angering
God.
An example of an apostate in regard to the entire Torah is one who
turn to the faith of the gentiles when they enact [harsh] decrees
[against the Jews] and clings to them, saying: "What value do I have
in clinging to Israel while they are debased and pursued. It's better
to cling to those who have the upper hand." Such an individual is an
apostate in regard to the entire Torah.
