I don't understand the decision not to use CoreXY. I have a very very similar plotter that also cost me next to nothing using essentially the same BoM as this. It's directly descended from this one [0]. It can absolutely fly, I've spent some time tweaking it but I can reliably and accurately drive it at something like 20cm/s. That's largely a factor of the low inertia of the CoreXY design, which keeps both the main steppers stationary during operation.
Anyway, plotters are dope and to anyone thinking of building one of these, I highly recommend a CoreXY based design.