My tips on PARTITIONing:
http://mysql.rjweb.org/doc.php/ricksrots#partitioning
Given that you always have something like
WHERE account_id = <constant> AND ...
then partitioning on account_id is no more efficient than having all of the indexes be "compound" and start with account_id. This is regardless of the eventual size of the table.
Also, your table will barely meet the second bullet item, since it will grow to only 3M rows.
> If there are any potential down sides to doing this (even if the up-side might be small)
Both the up- and down-sides are tiny. So, why add the complexity?
You should, however, use ENGINE=InnoDB and have innodb_file_per_table=1 before CREATEing the TABLEs. (It can/should be fixed later, but at a cost.)