Nobody is going back through thousands of product pages adding contextual links by hand, and asking a team to do it as an ongoing task guarantees it gets skipped the moment deadlines pile up. It needs to be structural, not a task on someone's list.
Breadcrumbs that reflect real category hierarchy, "related products" sections driven by actual attribute matching (not random), and category/collection pages that genuinely link into their best subcategories all create internal linking automatically, without manual upkeep per product.
Your highest-value pages — top category pages, cornerstone blog content, the products actually driving revenue — are worth manually curating internal links for specifically. The structural fixes handle the long tail; manual effort should concentrate on what matters most.