table_open_cache won't increase above 431 - MariaDB 5.5
Based on Open_tables being 431, I assume that you have only 431 tables in all the databases?  Or at least only that many are in use?

What is the value of table_open_cache?

What is the value of Table_open_cache_misses / Uptime?  If that is less than, say, 1/sec, then the table_open_cache is "big enough".