Would be really useful to have

$this->db->table('table')->replace()

It is the same like INSERT, just with REPLACE is used

Or

$this->db->table('table')->inserOrUpdate($newData, $updateData)

Useful when update should for example increment some column, change date, etc.

Reference:
https://dev.mysql.com/doc/refman/8.0/en/replace.html
https://dev.mysql.com/doc/refman/8.0/en/insert-on-duplicate.html