How do I unlock the "All Skills Learned" trophy after hitting Level 50?

The main quests as well as the Hunter Trials will award you Skill Points. If I remember correctly, main quests usually give you a Skill Point as a reward. Getting Blazing Suns on all 3 Hunter Trials at a Hunter Trial area will also award you a Skill Point. Additionally, completing Cauldrons and climbing Tallnecks will reward you Skill Points. If you haven't done all of these, you definitely should. From experience, I capped out my Skills by the mid-40s and was still receiving Skill Points from main quests and level ups, despite having nothing to spend them on.

By the end of the game, completing Hunter Trials and Main Quests should award you way more than the required number of Skill Points required to unlock all skills.