A long-lasting interorbital nuka-ferry, otherwise you'll be mostly fueling the fuel ferry.
Wait... Then why fuel? Just call it "tug" and taxi the capsule along the Earth-Moon-Earth route.
...But they why dock/undock the capsule. Make the capsule a part of the tug and taxi on the ISS-LOPG-ISS route.
So, just let it be an interorbital bus carrying the passengers delivered to ISS by a reusable Dragon/Dreamchaser/whatever.
Let it have it's Orion capsule as an escape pod (but don't land it every time) to abandon the tug if something went wrong.
And let it carry fuel just for a reusable lunar lander. Also the lunar fuel is additional rad protection for the crew, so no need in additional tug.
Imho, until long-lasting nukes, any lunar program will be just a hull test with poor lunar landing abilities. Particularly, LOP-G.
Imho the only purpose of LOP-G is exactly a hull&equip testing of a future long-term (Martian?) ship, with limited lunar applicability.
Sounds like "They are speaking about the rear-side Chinese rover, let's remind that we are still here, too."
Or a fully-reusable lander with replaceable fuel tanks for landing, and inner one for ascending.
They can vent and detach the tanks and use for their lunar needs.