1. Starship is never going to be usable for a Moon mission.
2. There's little scientific value in Moon missions.
3. There's never going to be long-term missions to the Moon.
I maintain all three simultaneously.
One of many wacked-out things about the plan.
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.
A single trip launch will always be constrained like this due to the tyranny of the rocket equation.
A modular mission system with multiple launches is the best way to expand capabilities and enable things like landing larger payloads for more advanced or long-term missions.