I think your vision of Mars is based on Hollywood.
The Sabatier reaction is inefficient; as Wikipedia notes, it requires 17 MWh to produce a single ton of methane, not including the energy costs associated with electrolysis of local water sources for the H2. Hydrogen represents about 1/4 of the weight of methane, and so you'd need another roughly 0.4 MWH for the electrolysis, for a total of 17.4 MHh to produce 1 ton of methane.
Each MWh is roughly the energy needed to power 1000 homes. That's not some "largish radioisotopes." That's a full-scale power plant. We don't have many power plants that can produce that sort of output but which are light enough to launch into space or simple enough to be assembled on-site. Solar (the easy option) would require a minimum of 15-20 acres on Mars, and approximately 120 tons of solar panels, not including wiring and other supporting infrastructure. We don't have any spacecraft capable of taking that much weight, so that's multiple orbital launches and multiple spacecraft just to get the solar panels to Mars, and we haven't even started discussing the weight or other equipment needed to get the solar panels down to the surface, let alone transport the habitat modules or other equipment, or the astronauts and colonists making the journey.
TLDR: Mars is a pipe dream with current technology.
I assume you're basically trolling here, but as I mentioned, obviously the radioisotopes would be for emergency power generation - life support in the highly improbable case of all other power sources simultaneously failing, not as a driver for industrial level manufacturing. You're also far off on solar estimates, probably in part because that Wiki page hasn't been updated in well over a decade and solar tech has rather change in the interim. You're looking at ~590W/m2 solar irradiance at Mars' equator, so production of something like ~100W/m^2 with typical consumer panels and perhaps 150W/m^2 with high end panels. So that's in the ballpark of ~0.1 acres for a MWh of production.
Hollywood, so far as Mars is concerned, is mostly based on the Martian which is a hard sci-fi and phenomenally well researched book. The mistakes it made, inadvertently and intentionally, only make Mars colonization even easier than demonstrated. For instance the raging dust storm of the movie (and book) does not exist (and was an intentional fib). Low atmospheric pressure means the most fierce dust storm would have all the force of a very light breeze. And similarly his adventures to extract water from the rocket fuel were completely unnecessary as it turns out the seemingly barren regolith is surprisingly moist at 2-11% water by mass, an unintentional mistake as this was only discovered after the book was published.
The Sabatier reaction is inefficient; as Wikipedia notes, it requires 17 MWh to produce a single ton of methane, not including the energy costs associated with electrolysis of local water sources for the H2. Hydrogen represents about 1/4 of the weight of methane, and so you'd need another roughly 0.4 MWH for the electrolysis, for a total of 17.4 MHh to produce 1 ton of methane.
Each MWh is roughly the energy needed to power 1000 homes. That's not some "largish radioisotopes." That's a full-scale power plant. We don't have many power plants that can produce that sort of output but which are light enough to launch into space or simple enough to be assembled on-site. Solar (the easy option) would require a minimum of 15-20 acres on Mars, and approximately 120 tons of solar panels, not including wiring and other supporting infrastructure. We don't have any spacecraft capable of taking that much weight, so that's multiple orbital launches and multiple spacecraft just to get the solar panels to Mars, and we haven't even started discussing the weight or other equipment needed to get the solar panels down to the surface, let alone transport the habitat modules or other equipment, or the astronauts and colonists making the journey.
TLDR: Mars is a pipe dream with current technology.