I wrote this post because what you are currently doing is setting bad precedent.
You tried to refuse payment on the basis that the UI was not good enough. When I asked what makes a UI good you told me you essentially had no idea.
And then once you finally agreed to pay (due to different reasons), you didn’t for the next 2 weeks and when I asked for an estimate, only gave me “this year”, which felt like you wanted to stall this out until I gave up.
I expect that 2 weeks is a pretty reasonable timeframe for paying for finished work, and I would have been fine with waiting longer if you gave me proper justification.