A pillar page's job is to comprehensively cover a broad topic, which makes related-term research foundational to deciding what sections and subtopics the pillar should actually include.
Use Related Terms to Define the Pillar's Scope
A thorough related-term list gives you a clear sense of everything a comprehensive pillar page on the topic should realistically address.
Turn Related Subtopics Into Sections
Each major related concept can become its own section within the pillar, giving the page a natural, comprehensive structure.
Link Out to Dedicated Pages for Deeper Related Topics
Where a related subtopic deserves more depth than the pillar can reasonably provide, link out to a dedicated supporting page instead of over-extending the pillar itself.
Next step: Use the LSI Tool to map out the related subtopics your pillar page should be structured around.