Due to time limitations, this is all off the top of my head rather than citations or back of the envelope numerical calculations, apologies in advance.
So you're fixing solar activity, Earth's rotation and Earth's magnetic field, but allowing everything else to proceed. After nigh infinity years, the fate of Earth is unknown due to limitations to our current knowledge of physics... I'll take the liberty of adding a few additional assumptions: that the expansion of the universe remains "on its current course" and we don't have runaway acceleration into a "dark rip" scenario, and that protons do not eventually decay. One possible outcome could be that the entire planet eventually converts itself into a ball of iron due to quantum tunnelling, and then into neutron planet (not if sure there's enough mass, and not sure about the weak nuclear equilibrium) and then a tiny black hole due to more of the same, which then evaporates into photons via Hawking radiation. These would take a cosmically absurd amount of time.
However, you seem to be more interested in earlier events that take mere billions and trillions of years, particularly the cessation of plate tectonics. Current speculations include the idea that Earth might lose all of its oceans in a billion years, at which point plate tectonics ceases due to reduced lubrication. However, these models account for increased atmospheric escape due to the sun becoming hotter. Since you're holding solar activity constant, I suspect this process will take considerably longer, and it's no longer clear to me that tectonics will continue until Earth loses its oceans and atmosphere and their erosive processes.
When tectonic activity does cease at some point, I don't think it's clear that continental formation will cease immediately. The planet could still have enough internal thermal energy to drive ongoing volcanic activity, only instead of plate edges, now it's more of a haphazard "hot spot" pattern like on the planet Venus. Volcanic activity of the "flood basalt" type could still form new continents. Once the enough energy has been lost for the volcanic activity to end, that may also be the end of the carbon cycle. If the Earth still has an atmosphere at this point, all of its carbon content may end up in rocks, leaving nothing for photosynthesis, leading to the end of all life as we know it. TLDR, long before it's an iron ball, I think the Earth will be a lifeless rock, devoid of ocean or atmosphere, but possibly not eroded to be completely smooth (furthermore, without erosion, new craters from astronomical impacts tend to stick around).