we *do* know - now - that the first flights to Mars were in the 2030's ("One Small Step").
A common misinterpretation. "One Small Step" never claims that Ares IV would have been the first flight to Mars. For all we know, it was the nineteenth.
After all, it was stated in "Space Seed" that interplanetary travel was routine by 2018 already, and that the first designs capable of such journeys dated back to the 1990s if not earlier and were already retired decades before "One Small Step". It doesn't make sense that these journeys would all have been to Venus and none to Mars - or to Saturn.
Also, "The Changeling" establishes an interstellar unmanned probe for 2002, making it fairly incredible that an unmanned probe to Saturn would have to wait until 2009.
And it would be completely inconsistent with other Trek predictions (including the one in that very same episode, namely of a Moon landing in 1968 or 69 or so) if uncrewed missions to Saturn would be delayed to the 21st century. Back in those days, everybody who believed in the Moon program probably also believed we'd be hopping around the surface of Mars by 1984 at the very least.