Question

Hello Rabbi,
It is well known that the rule is: one who is engaged in a commandment is exempt from another commandment. At first glance, it is difficult to understand what Maimonides rules (Laws of Torah Study 3:4): “If one had before him the performance of a commandment and Torah study, if the commandment can be performed by others, he should not interrupt his study; but if not, he should perform the commandment and then return to his Torah study.”
But Torah study is itself a commandment, so how is it different from other commandments, such that it must be interrupted in order to fulfill other commandments? Why do we not say here too that one engaged in a commandment is exempt from another commandment? Perhaps one could infer from here that there is in fact no commandment to study Torah beyond the minimum measure of reciting the Shema morning and evening?
With blessings,