>When approached by NASA in 2018 for potential SLS replacement, they stated to NASA and press questions that the best theoretical max for Falcon Heavy disposable TLI payload is 18,000kg, but only a realistic 16,000kg to lunar orbit for a crewed vehicl

Source?

>The Mars injection is using the 6 month Holmann transfer window,

Even the lowest energy Mars transfer still needs more energy than fast TLI.

A best case Hohmann transfer during an ideal window like the 2033 window would be about 3500m/s. However, the upcoming 2024 window will be more like 4100m/s. On average it tends to be around 3900m/s, so I expect SpaceX's payload figures to be based on something like that.

By comparison, Apollo's fast TLI burns were nominally 10,400fps, or ~3170m/s. Call it 3200m/s. No matter how you cut it, the Mars transfer needs several hundred m/s more delta-v.

It seems very odd that an extra ~700m/s to get from GTO to TLI reduces payload from 26.7 tonnes to 16-18 tonnes, or a whopping 10 tonnes less, yet another ~700m/s to get to TMI results in virtually the same payload.

Even if SpaceX are using the best case Mars transfer, you'd still expect it to be a few tonnes less.