The Earth doesn't receive enough radiant energy from the sun to enter a Venus-like state from increased atmospheric CO2. At least not without waiting hundreds of millions of years for the sun's energy output to increase, in which case you might as well just choose one of the high-patience destruction options.

As a much nearer term goal, the Earth could be rendered all but sterile by removing and permanently sequestering carbon from the atmosphere to break the carbon cycle [0].

1) Direct air capture of CO2 from the atmosphere.

2) Conversion of CO2 to carbon with electrolytic hydrogen [1].

3) Reaction of carbon with silicon dioxide in an electrical furnace to form silicon carbide [2].

4) Surface storage of ever-increasing quantities of silicon carbide to end the Earth's carbon cycle and life itself.

Bulk silicon carbide is effectively inert to biological or geological decomposition under surface conditions on Earth. It can break down if subducted into high temperature regions of the mantle, so it will need a watchful eye to keep geology at bay until the sun's expansion destroys the Earth completely.

You'll need some basic clanking replicators (e.g. strictly earthbound von Neumann machines) to accomplish this. 99% of the life-elimination process can be finished in mere thousands of years. Either nuclear breeder reactors or paving the world's great deserts with solar panels can provide enough energy for the project.

Silicon carbide can also oxidize in a furnace, so human civilization must be neutralized. But if you have a fleet of von Neumann machines dedicated to eliminating life that's one of the first problems to solve anyway.

"Won't falling atmospheric CO2 trigger a new glacial maximum and greatly retard the rate at which environments can be decarbonized/sterilized, once ice cover impairs carbon cycling to the atmosphere?"

Great question!

You don't want a lot of frozen water on Earth because that will make it harder for CO2 to circulate through the atmosphere where it's easy to capture. It takes thousands of years for the ocean to fully overturn and exchange with the atmosphere under present climate conditions; there's no sense in making it even slower with sea ice. Nor is hiding soil under glacier cover a sustainable solution to the life problem. But you also don't want to leave CO2 in the atmosphere just to keep things warm and freely circulating.

Some small portion of the replicators' effort can go toward manufacturing potent non-carbon-bearing greenhouse gases such as nitrogen trifluoride or sulfur hexafluoride. They are so much more potent than CO2 that just a bit added to the atmosphere should keep the Earth comfortably warm even as CO2 becomes too scarce to support photosynthetic life.

In only 10,000 years most multicellular life can be eliminated by carbon starvation. Some remnants of life will remain deep underground and near hydrothermal vents on the ocean floor. The machines must continue to eliminate CO2 as it is emitted by natural geological processes and maintain the mountains of silicon carbide so that they remain intact into deep time. Otherwise there could be another large scale outbreak of life before the sun finishes Earth.

[0] https://www.pmel.noaa.gov/co2/story/Carbon+Cycle

[1] https://en.wikipedia.org/wiki/Bosch_reaction

[2] https://en.wikipedia.org/wiki/Silicon_carbide#Production