MariaDB: Created_tmp_disk_tables high after start
You may have other limiting factors involved. Here are some suggestions:
Additional Buffers
The per-connections buffers you may need to increase includes

join_buffer_size
sort_buffer_size

Queries will demand temp table based on join and sort operations. These may need to be increased.
Improve Queries
Queries that need to do joins and sorts can be reduced if queries had proper indexing. I would suggest that you profile all your queries using pt-query-digest and create whatever indexes that table need to reduce the incidence of temp table creation.
Create a RAM Disk
You should map a chunk of your RAM to be a dedicated RAM disk. Then configure tmpdir to point to it. See my post Is it bad to create many mysql temporary tables simultaneously? on how to set one up.