Chasing a balance is a process, not a person
If one coordinator holds every due date in their head, the process leaves when they do.
The quiet dependency
Ask an events team who chases outstanding balances and you will usually get a name rather than a system. That person knows which couple pays late, which card bounced in the spring, and which booking has a note in the margin about a family paying in thirds.
It works until they are on leave in the week four balances are due, and it stops working entirely when they leave for good. The spreadsheet survives; the judgement around it does not.
What to write down first
Before any tooling, write the rule you actually follow. Most teams discover theirs is a sentence: twenty-five per cent on booking, the rest in equal payments ending fourteen days before the date, and a call rather than an email after the second failure.
Once the rule is a sentence it can be a schedule. Once it is a schedule it is the same for every booking, and the handover is a login rather than an apprenticeship.
Keep the exceptions deliberate
Every venue has bookings that do not follow the rule, and that is fine — a schedule you can override is more honest than a policy nobody keeps. What matters is that the exception is recorded against the booking instead of remembered by one person.
One number worth watching
Not how much is outstanding, which moves for ordinary reasons, but how many payments failed and were still unpaid a week later. That is the number that turns into an awkward conversation on the day of the event, and it is the one a schedule with retries and its own emails is actually able to move.