While this project has some serious issues, especially with regards to payment and actual completion, there are larger issues for all of us to consider in this case, which is "the client". We don't know what your brief was, but you exchanged a slow-drip of emails of design tweaks and gear changes for over a year. To me this sounds like a client who doesn't know what they want. I too would be hesitant to start welding if I were them. From late spring 2018 to mid autumn the both of you essentially went quiet on the project, until you decided that it was time state an end date. Now setting aside that you suggest the framebuilder may never do it - which if true this framebuilder is a charlatan and you should demand a full refund immediately- it has still taken taken nearly 2 years to not get a bike. If this builder is not a charlatan and has legitimate project-related reasons for the delay, then the non-bike is both yours and the frame-builders responsibility, for reasons I'll detail below.
Framebuildres are not project managers. If we want a deliciously well-managed build with inputs into colourway and subtle design options delivered on schedule and to your door, I would suggest a person get themselves to an Audi dealership. However these are sole-traders (generally weirdos) working on commission for clients who start out not knowing what they want, what it might eventually look like or when they want it by. The difficult hometruth here for anyone commissioning a bespoke bike is that it's going to be your job to make it straightforward for them to make your bike. If a project is dragging on and on and on it's probably because you let it. Set some dates for completion, which if not met then the project is cancelled or adjusted otherwise amended / refunded. If someone changes their mind mid-project it's going to fuck the timeline up guaranteed.
We think we live in this age of consumer experience with back massages at a cafe that's open until 8pm and Amazon next-day delivery handed to us at our door by a puffed-out van driver. A bespoke bike is probably made by some former crustpunk stranger via a massive injection of someone else's capital in a souped-up garage. This framebuilder will not be qualified in PRINCE2. I wonder at people's expectations when commissioning bikes.
Think of this as food for thought generally instead of a direct dig at the OP.