The essence of this machine is the big bed and the automatic toolchanger. This way we can press start and do other usefull things while the machine works completely on its own. There's no need to check it every 10 min and change the tool when neccesary. You are absolutely correct that you can get a milling machine for €2-300 (see aliexpress) but we want to make it a machine between the ultra low cost (and low #of features) and the unaffordable professional (LPKF) machines. If we look at the othermill (now property of bantam tools) https://www.bantamtools.com/collections/machines/products/international-bantam-tools-desktop-pcb-milling-machine-with-advanced-bundle we see that their most basic machine costs 3200 USD. This machine has no automatic toolchanger and smaller bed.

That feels very, very expensive to me. Maybe it's my ignorance of the market but I was expecting something closer to €200 or maybe €300. Who is the customer? If it's the hobbyist who fabs up an occasional PCB, even a few week, I think that's going to be very difficult sell at €5000 as one can send out for quite a lot of bespoke work for that price. Even €3000 for the kit feels high when a knock-off i3 clone 3d printer kit with a 200mm^2 bed and lead screw drives can be had for €200, delivered to my doorstep which, at a distance, looks like a substantially similar machine. The difference seems to be a cover, milling head, and tool change mechanism.
Think on the ways to get the costs down and build on others' economy of scale. I would set about starting with a 3d printer and modifying it to suit, if it were me. Would a cable-driven dremel work? A Bosch router? Could you make the tool changeover manual in order to simplify the design?