The ITS is planned to use 10-tonne metha(l)ox-gas thrusters for RCS, fed from the autogenous pressurization coolant loop of the Raptor.
The dev Raptor is smaller than the final design, at around 1000 kN SL thrust; I'd project around 1150 kN vacuum-optimized thrust.
If you built a methalox upper stage for Falcon Heavy based around two of the dev Raptors and gave it a few of those gas thrusters (dual thrust-axis lander design), then refueled it in cislunar space, it could perform most of the landing burn using its main engines and just use the gas thrusters for the last little bit of the burn. It would have no trouble carrying enough fuel along with it to return to lunar orbit, or even to an Earth entry trajectory.