Average Days to Pay - Quickbooks Online Data (QBO)

Analyzing a company's data in QBO and unfortunatley the payments don't have an invoice number assoicated with them (apparently on QBO this only happens if the user puts the invoice number in the payment memo, which is wild for a company with the accounting history of Intuit).

I'm seeking to understand the average days to pay it takes for each individual customer at this company. How long is one invoice outstanding before that amount is collected? I've been banging my head against this for a couple weeks and I'm not coming up with a solution.

It's made more difficult as many invoices are the same amount, or different amounts, and payments are either full payments, payments for multiple invoices, or partial payments. It's all over the board so there's no simple lookup or matching to apply.

The best I've done is been able to determine when the account was zero, but that's not necessarily the answer. It's possible a customer always keeps a balance on their account, even though they're paying an invoice off in 32 days on average or something of that nature.

If an invoice is $200, how from this data can we identify when $200 was received? How many days was that? And if the next invoice is $300, how can we identify when $300 was received? And then how do we average that for all the invoices for the customer?

There are some repeat invoices, which I've indentified in the last column. Luckily most of those are from 2018 and in 2020 (the time period I need) they're clean. I didn't want to delete the old data, though as the balances can flow into each other.

It's a fun puzzle, but I'm spinning my wheels on it. Appreciate any advice/wizardy someone can up with in the example file. Thank you!