I'm self employed and whilst not in the building industry the theory is still the same. Industry standard and contract rules decide when you pay. I've had the tinkly laugh " Oh I'm so sorry I'm out / away from my laptop/ just so terribly busy " and it immediately ensures I will never contract with them again.
Your personal sensibilities that you felt ick because you might not pay were spot on. If someone flits about not taking it seriously and not doing a 5 minute bank transfer absolutely flags them as not likely to pay. You did that to yourself.
Now as most do I keep a buffer for non payment but thats etting harder and harder with the cost of living. Customers faffing about with being out or busy will absolutely get you banned form most future lists I know that work self employed. I will also say from a human component it's unutterably arrogant. Your time is more important than their income?
Pay the man. I don't chase like this, I will send another email 48 hours after the date I expected payment but by that time I've already flagged you in my customer list. 5 and 7 days later I send a reminder email quoting my terms and conditions. By that time I will never contract with you again and I've already started planning the route to court to get my payment. I have no hesitation using legislation to get my payment.
Don't forget self employed are often networked within their industry. People who are flaky over payment very quickly get a reputation. In my business one big company had a manager who merrily and tinkly laugh kept claiming to several trainers she was just so terribly busy to confirm invoices to her finance department. All the local freelancers as in the entire quarter of the country ) stopped agreeing to work with her department after she delayed a month confirming these for 3 trainers (different specialities) in a row . (It was a company so more Lee way is given than when for a single person). Word got around. Fast.
Everyone refused to contract if it was her team. She got pulled and given a warning for losing them access to that training. Tricky as our specialities mean there are not that many people who can do the training. She eventually got demoted (for that and to be fair many other things). Now I grant you this is a company but the rule stays the same. Also with new customers and often in my case small services run and owned by a single person get very little buffer room because if you do that to me on the first job I will not trust you going forward.
Don't be flaky with payment if the job was done and done well. Your personal schedule is irrelevant.