Both previous_post_link and next_post_link accept a 3rd boolean parameter $in_same_term, which specifies whether link should be in a same taxonomy term.
So in your scenario you would be looking to make the following update:
<?php previous_post_link('%link', '⟪ ⟪ ⟪ %title') ?>
updated to
<?php previous_post_link('%link', '⟪ ⟪ ⟪ %title', true) ?>
and next_post_link
<?php next_post_link('%link', '%title ⟫ ⟫ ⟫') ?>
updated to
<?php next_post_link('%link', '%title ⟫ ⟫ ⟫', true) ?>