I should like to state that I was also looking for case-insensitive tag-based search suggestions and have found that jmiba’s approach is the one that works.
In a different thread, it was proposed to simply copy the fields “tags_string” to “sort_string”.
Alas, this does not work, as “tags_string” is multivalued and sort_string is not.
So go with jmiba’s appraoch, it will work.