It would be interesting to see if this tech was viable for dev boards. i.e. when you want to design a new 2nm chip, what if you were using electron beam chips to test out designs?
E-beam lithography is used for research purposes all the time, so yes it's already used for "dev-boards".
That said, apart from the economics (it is very slow), you are also constrained with respect to the size of what you can write, the writing fields is typically quite small, if you want to make a chip larger than that you need to stitch fields together and you have to deal with stitching errors (which becomes more and more difficult the smaller your structures are).
That would almost certainly be more expensive. When people talk about modern masks being $15M or so for each mask set, a huge fraction of the cost is this process for the masks.
It also doesn't tell you as much about how your design actually runs on the process in question.
That said, apart from the economics (it is very slow), you are also constrained with respect to the size of what you can write, the writing fields is typically quite small, if you want to make a chip larger than that you need to stitch fields together and you have to deal with stitching errors (which becomes more and more difficult the smaller your structures are).