The problem with not having a separate table; is that a logging field is going to be impossible to interrogate for data that is reportable. You can use automations to enter a value in your main record; and use that data to create a child-table record to store the payment information; or you can pre-build a table of 12 month's of records (or for the remaining calendar year); and then update the month-records when the payments are made.
I do not think you will get a reportable-solution from a single-text logging field; a two-table structure is the best way to go; even if you decide to have a table that stores a Year-value; with 12 monthly payment values in a single record. This would still be a 2nd table.
Do not be afraid of tables and relationships! They are what make Quick Base great!