MYSQL Multiple table optimization for advanced search
Don't use MyISAM, use InnoDB.  http://mysql.rjweb.org/doc.php/myisam2innodb
EAV is inefficient, especially when you normalize either of the columns.  http://mysql.rjweb.org/doc.php/eav  And read more about Entity-Attribute-Value schemas in stackoverflow.com
You are missing some critical indexes that are critical to performance.  http://mysql.rjweb.org/doc.php/index_cookbook_mysql#speeding_up_wp_postmeta
At this point, FOREIGN KEYs are the least of the issues.