It would be great if MariaDB SQL allowed for adding and ideally also removing/modifying individual enum items without having to specify all the existing items in the enum.

The syntax may be like this:

ALTER TABLE dog
MODIFY size ENUM ADD VALUE 'large';

ALTER TABLE dog
MODIFY size ENUM RENAME VALUE 'large', 'very big';

ALTER TABLE dog
MODIFY size ENUM DROP VALUE 'very big';

or something similar. We can borrow some syntax from PostgreSQL which has "enum types":

https://www.postgresql.org/docs/16/sql-createtype.html
https://www.postgresql.org/docs/16/sql-altertype.html

but adding CREATE/ATER TYPE is a more complex task.