The Motivator for Moral and Religious Life

Hello Rabbi,
I wanted to ask two questions regarding the reasons for actions and the reason for fulfilling duties(?) that are incumbent upon us, such as serving God or morality,
A. We assume that when a person acts, he does so because of a reason or purposeful motive (motivation in the Hebrew language). And of course the question arises as to what is the range of reasons in accordance with which a person may act. The issue of working for God's sake, obedience to the categorical imperative of morality, and more. But it seems simplistic to say that work does not belong to the realm of work for its own sake.
Because every reason a person acts is because of an internal and subjective reason of his own that was the motive for the action, such as some lustful interest or another. But it does not seem appropriate to do it because of a reason external to him, because otherwise the question arises why a person would act not to fulfill his desires.
on. Also, I wanted to ask, even if we accept that there can be an external motive, what is still the motive for the existence of morality, even if we are convinced of the existence of morality as an idea external to us, what gives it the validity that we are convinced to obey it? For example, why is it similar to the fact that the Five Commandments are engraved on Mars or that you found an old book with instructions on how to behave? In all these places, it seems unreasonable to refer to the written laws just like that, because the validating factor for that object is missing.
And therefore, if you claim that God is the one who created the moral laws, then the question also arises regarding Him: why should I do God's will?
And at this point, it splits into two:
I) If you argue that the reason for doing God's will is "this way" – then even though we must reach an infinite regress, there is still something uncomfortable about this concept because it seems a bit arbitrary, for example why wouldn't we be convinced to do the duties written on Mars, or those written in the book we found, and do them just "this way."
II) And if you claim that the reason for doing God's will is because of another moral motive such as gratitude, the question arises about this motive in my opinion. First, you contradict your previous statement that morality comes from God, and even if you claim that gratitude is a motive that is its own cause, then it could already be argued in the first part – I – that obedience to God is its own cause. But then you return to our question there.
To conclude, it seems that we must say that serving God does not belong for its own sake, but rather for an internal purpose for us, such as the world to come.
But on the other hand, we feel that the moral commandments such as "Love your neighbor" are indeed true! Despite all the nonsense said here,
Therefore, since God, the Holy One, wanted us to uphold the values ​​of justice and religion, He therefore bestowed upon them a good reward in the future, as it is said, "The Lord desires for His righteousness."