How to replace the existing metatag using the backend to insure a thumbnail image gets fetched when we share on social media?

add_action

add_action currently supports 4 parameters: string $hook_name, callable $callback, int $priority = 10, int $accepted_args = 1

Try adjusting the 3rd parameter “priority” to add the action later than any existing actions (which may have been created by plugins or themes).