As a rule, you'd only use bookmarks with a TOC field if you're wanting to restrict the TOC content to headings in the bookmarked range. Bookmarking cell content & referencing bookmarks in table cells can lead to anomalies such as the reference replicating the entire cell (not just its contents). Coupled with the fact bookmarks are:
• easily deleted by accident;
• require careful editing to ensure intended additions fall within their range; and
• are prone to being extended by accident to include unintended content,
they're not a good choice for what you have in mind.