Ask your accountant if writing off older debts (anything over 12 months) gives you a tax advantage. Each business is different. e.g. invoice basis GST
Then pass those debts onto a debt collector. Why waste your valuable time on them.
Keeps your books clean by reducing aged debtors size, and will cost you nothing.
Any successful collections come back onto your books as a journal adjustment, so still nice and clean.
Now’s the time to do it. Before EoFY.