> It's at least half-a-decade to a decade away from initial reusable testflights, and perhaps more given the under funding of the Artemis program
SpaceX is not dependent on Artemis funding other than lunar Starship. Yusaku Maezawa's Dear Moon also provides $500 million in Starship funding (presumably tranches get released for successful milestones including this test).
SpaceX are otherwise self-funding the Starship test program so far. It might become easier as Starlink satellite internet system comes out of beta and generates large cash flow.
Elon Musk estimated the entire Starship program to cost a mere $10 billion. Taxpayer money helps (especially for moon and Mars missions), but SpaceX needs to create a next generation rocket (both stages fully-and-rapidly re-usable) to stave off future competition (though Blue Origin is very far behind).
Even without government funding for Mars and the moon, low-earth orbit provides ample incentives: commercial, government (ISS etc) and military (reconnaissance sats) for SpaceX to slowly and steadly develop Starship. A few billion for Starship development for moon missions would definitely accelerate the otherwise self-funded schedule to land unmanned payloads on Mars from 12+ years to 5 years.
Also Starship is not 5 years away from orbital flight but less than 18 months away. Super Heavy is built on exactly technologies as Starship (mass-manufacturing stainless-steel rings then rapidly stacking them): the first test articles are already being assembled using the same equipment that Starship uses. The Starship re-entry heat shield tiles are the biggest unknown at the moment, but future test flights of SN9 and beyond will figure this out.
The bulk of Starship funding is not coming from NASA. Well maybe if you consider SpaceX re-investing the profits generated from the International Space Station crew and cargo resupply contracts as "NASA funding" (but I don't).
SpaceX has received some development funding for the Raptor engine ("In January 2016, the US Air Force awarded a US$33.6 million development contract to SpaceX to develop a prototype version of its methane-fueled reusable Raptor engine for use on the upper stage of the Falcon 9 and Falcon Heavy launch vehicles, which required double-matching funding by SpaceX of at least US$67.3 million.").
EDIT: As far as funding goes, I believe the direct Starship funding is NASA's Human Landing System: Funding $135 million of DESIGN WORK as part of the Artemis program over (10-month period ends in Feb 2021) for the Lunar Starship variant. The Air Force did provide $40.7 million for developing the Raptor engine a few years ago though. It would appear that a significant amount of Starship investment so far as been re-invested profit, external investment and funding for the private "Dear Moon" mission.