Make this Fast and Queryable

I have 100,000+ registered users on my WooCommerce site.

I have a subscription billing that people earn 5 entries each billing with. All users have the ability to earn entries for every $5.00 spent.

The entries are stored by contest... (A custom post type)

A Single usermeta field containing an array records entry data in the following format.

array('CONTEST_ID' = 'ENTRY_COUNT'); 

or for example

array(1302 = 5, 12053 = 25);

I want to change this to be queryable in some format and store the reason for gaining entries. An example would be

array('CONTEST_ID' = array('reason' = 'initial_signup', 'type' = '+', 'ammount' = 5)
'CONTEST_ID' = array('reason' = 'refunded_order', 'type = '-', 'ammount' = '12'
);

I need to make the entire sequence quickly computable to get a given contests total amount.