Show all tags NOT applied to post

if i have understood your question, you want all tags from a taxonomy not applied to your post?

maybe you can use get_terms('your_custom_taxonomy') to get all of your tags,
and parse/diff to your current post tags to get all tags not applied to your post
https://developer.wordpress.org/reference/functions/get_terms/

or maybe try to use a filter like list_term_exclusions with your current post
https://developer.wordpress.org/reference/hooks/list_terms_exclusions/

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