Answer

In any case, the matter is almost explicit in the passage in Nedarim 8a: beyond a chapter in the morning and a chapter in the evening, the rest is optional (and therefore an oath can take effect on it). True, the Ran there writes that this is an obligation derived from exposition, and therefore an oath takes effect on it, but the Rosh there understood it in its plain sense. See also the passage in Menachot, which is explained in my article here.

Of course, the intention is Torah study beyond the minimum of a chapter in the morning and a chapter in the evening.
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Questioner:
A quote from the Rabbi's responsum: “The commandment of Torah study, beyond a chapter in the morning and a chapter in the evening, is according to many an existential commandment.” Please tell me who they are.
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Rabbi:
I mentioned this in my previous answer: all those who explain the Talmudic passage in its plain sense (unlike the Ran). See also the article I linked there, where I explained that even those who speak about neglect of Torah study and an obligation to study all day are not necessarily discussing the definition of the commandment of Torah study, but rather the matter of Torah study itself.