Return payload constraints are probably from using Orion as the return vehicle. Mass to the surface is much higher than Apollo since that is launched separate from the crew.
[EDIT] Apparently there are multiple plans involving even more spacecraft, because why not I guess? It's as you describe for Artemis III, but then gets way more complicated with Artemis IV, involving more spacecraft for some reason.
NASA has optioned an additional lander from Blue Origin but that would be taking the same role as SpaceX's lander, shuttling from lunar orbit to the surface and back to lunar orbit.