I did this for myself several times. I started with winforms and evolved into a full blown clone of ChatGPT.com experience built as a razor/MVC project in .NET10.

I have 2 versions that are just for working on unity projects. One that is for working on the client project. There is also one that I refer to as the ring of power / Death Star that I keep mostly to myself. Eventually, the client saw what I was doing and decided they wanted a custom ChatGPT.com experience for the whole team, so I got paid to build the most recent one.

Using AI to assist with the development of this stuff is the RSI you've been looking for. I'm in a situation where I was only able to solve a problem due to two levels of bootstrapping enabled by custom harnesses. The product the client uses cannot be "one shot" using the latest frontier tools. I had to build an ad-hoc agent loop fit for just that one purpose using another tool that was possible to build via copilot. For example, using copilot in VS over a unity project's scripts might as well be a drunk person throwing darts from a boat compared to a purpose built harness that can execute ad-hoc scripts in the editor environment as often as it pleases.

The client's custom experience is better than ChatGPT.com in many important ways now. It certainly performs better. It will run on safari without doing horrific things to layout and my CPU. You can easily exceed the capabilities of the frontier AI companies in ways that are important if you use their tools cleverly. These companies are great at making AI models. Not harnesses, websites, apps, etc.

Could you share what the custom harness does better? I'd also like to build my own custom harness and any tips/tricks that aren't already widely known would be very welcome!