The light source is the “easy” bit. The mirrors, masks, and the rest of the machine are all individually as difficult if not more so.
The wafers have to be positioned to nanometer accuracy repeatedly and at high speed! It’s hard to believe that’s even possible, let alone commercially viable.
Managing the light source, specifically the 13.5nm length on the wave spectrum, that gets generated from overheated tin plasma, is in fact the most challenging part of the machine. Here "managing" includes the process of hitting a rightly sized tin droplet with lasers at the right angles, and all the rest of the complicated fluid math necessary to get the most of that precious lighting moment, as well as the proper handling of that spark event's after-effects, of course. As opposed to the rest of the machine parts (like directing the EUV light to the reticle through those mirrors you mention), the light generation part is dynamic, very easily to get wrong, and very costly to iterate on.