If your enum has 8 values, it stores in 1 byte, if it has more than 8, it stores it in 2 bytes.

You're probably thinking of the SET type, rather than the ENUM type.

You also had to be careful to include `ALGORITHM=INPLACE, LOCK=NONE;` in your `ALTER TABLE` statement when changing the enum or it would lock the table and rewrite it.

This is a common misconception; that's not how ALGORITHM=INPLACE, LOCK=NONE works. An ALTER TABLE without ALGORITHM and LOCK will, by default, always use the least-disruptive method of alteration possible.

Adding those clauses in MySQL just tells the server "fail immediately if the requested method is not possible". The semantics in MariaDB are similar, just slightly different for INPLACE, where it means "fail immediately if the requested method or a better one is not possible".