MySQL/MariaDB Read only except for a single table
I don't believe you can do read_only = 1 (except for table x);

So I would think your best option would be to set all your users to read_only:

GRANT SELECT ON *.* TO 'user'@'1.2.3.4';

Then the users that need to write to that table would be given permission to write to just that table. e.g. 

GRANT INSERT, UPDATE ON 'dbname.tablename'@'1.2.3.4';