Are there any disadvantages to partitioning on financial year?
Answering your questions above:


In RANGE COLUMNS (datetime) key word COLUMNS is unnecessary. It is used to specify a list of columns, where as you have only 1 column. The difference exists: for RANGE (datetime_column) you can specify partition boundaries in datetime format, whereas for RANGE TO_DAYS(datetime_column) partition boundaries should be effectively in date format.
Partitioning on datetime is OK and it is very often.


Please refer to MySQL Manual, Chapter 18. Partitioning.

If the only reason for you using partitioning is structuring data, then may I suggest you looking at MERGE Storage Engine. It allows to create a table consisting of other tables. When queries to this overarching table have data only in some tables, then only those tables are used. It is easier to set up and maintain, than partitioning.