Hypothetical to wrap my head around scope: Say we had a spacecraft that was 100% capable of launching from Earth, orbiting the moon, and landing back on Earth, but it also had the docking equipment to be compatible with ISS. Could I decide, while orbiting the moon, that I'd rather dock with the ISS instead? Is that at all feasible or is it one of those, "we're not lined up for any of this. It's basically impossible." things?

It is one of those, "we're not lined up for any of this. It's basically impossible." things Completely different orbits, would need a ton of extra fuel, not at all possible for Artemis (or any other mission for that matter)

You’d need a load of additional propellant to insert yourself into the same orbit as the ISS on your return, which would have an exponential effect on the amount of propellant needed in the first place to get all this lot out to the moon. It would be a different vehicle.

Your comment is dead for some reason but let me help.

Orbital mechanics are not something you can just intuit, but are pretty simple.

Learn about Delta V. Play some Kerbal space program. View this diagram:

https://upload.wikimedia.org/wikipedia/commons/9/93/Solar_sy...

The ISS isn't listed but the math doesn't change much for it VS the listed 250km earth orbit.

TL;DR

In space you can think of yourself as captured in an orbit. It takes a lot of energy (fuel) to boost yourself out of that orbit and flying towards some other body. But after that, you have to also expend more fuel to actually be captured by that orbit. The returning craft would have to expend energy to be captured by the earth's orbit to rendezvous with the ISS, rather than just hit earth and land.