For SX to set up a base on the moon and mars, they will need 2 different types of landers: Cargo and Manned.
With manned, it is easy enough for ppl and supplies to be transferred from space-only to the lander. The question becomes, what about the cargo version?
So far, SX has shown a door on the side, but I believe that there is NO WAY that is going to work. The reason is that the cargo will be pulled out and then have to be stacked directly below the door. In addition, assuming that lunar and martian cargo systems never return to earth surface, then we need a way to move cargo from the starship leaving earth to the lander, or possibly something in-between.
With that in mind, about the only system that I can imagine would be to use the nose opening, docking nose to nose, and then having containers, moved from 1 system to the other. Finally, when landed, the nose is opened up again, and a crane on the inside is used to pull the container out and place it on the side.
What is interesting is that the crane could be attached to the rocket via a track that runs on the inside so that it can rotate around the lander and drop off containers directly to the ground as opposed to being forced to stack them.
Now, as to starship:
1) trivial for it to reach orbit. It was not because they are not working on orbiting, but on all of the stuff in-between such as take-off, separation, fuel pump testing, re-entry and landing on a single spot.
2) It is highly likely that SX will launch a couple of starlinks sats on this next launch. If not, it will almost certainly be the one after that. IOW, they will start using it for Cargo.
3) Re-fueling in orbit is not a big deal. They tested this in the last launch, internally. In addition, liquid transfers have been done for a long time (water, lox, fuel for ISS and other modules). They will need to build a tanker and a fuel-depot, but tanker will likely happen this year.
4) manned rating for launches will take a number of launches. Of course, that is exactly what SX intends to do with starlink cargos. Even if landings are not working yet, they will continue to use this to put up their starlinks that can not go on F9 ( though possible on FH ). The interesting item is how to test the life support. I suspect that they will be putting up 1 or more of these as space station module. Each 1 has more volume than the entire ISS (and far more than China's space station). Just by putting up the first one as a regular starship, they can sort things out for the ECLSS as well as layout, etc. Ideally, they would build several of these with later ones have multiple docking ports. That would enable these to quickly replace the ISS, while others continue to add their units to these for testing of their modules/ECLSS.
And yes, of this could be done before 2026.
Of course, that leaves the lunar lander. I would have to guess that once SX is able to land booster and starship on earth, they will have no issues landing on either the moon/mars. The only real issue will be (re-)entry into mars. The atmosphere is thin, but it is still there.