I have a customer who has not paid $38k of an 80K invoice - the job was from early 2022, my lien rights expired but what is my best course of action to receive payment
Hire a good construction lawyer and send a strong payment demand (assuming there are no legitimate defenses to payment).
A demand letter could work, but if they've owed you that much for this long (assuming this is a personal residence) then it's very likely that you will have to sue for breach of contract.