Time Investment Priority, Return
While Leo had spent over half a dozen years doing nothing but focusing on combat and physical growth due to the behavioral constraints imposed on him, the reality was that every minute spent inside any book was absolutely precious!
In a situation where he had access to the Heavenly Demon's Library at such an early point, the value of time had increased even further. Unfortunately, Leo knew that he had to wait till the time was right.
'Even being a fake genius wouldn't save someone from being called absurd.'
Choosing to learn the spear after displaying great affinity with the sword was already stretching it. If Leo had chosen to learn everything he got as soon as he exited the library, it wouldn't have been too far-fetched for him to be subjected to forced learning, or worse, he could be demoted to a normal rank member, become a servant, or even get thrown back to the street.