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I will say, imperial units are easy to divide by 6 and 12 ... and they're also entirely defined by metric units anyway (The lbs is defined by the kg in America). I've heard the US tried in the 60s and failed. I think a rollout would be fairly straight forward though:

1. All new federal Interstates or highways would require a new speed limit sign with MPH and KPH on them. Make them distinct in design. Have a / and have both units marked.
2. After 4 years, incentivize cars to put KPH on the outside and MPH on the inside. Make the design distinct so it's clear your gauges are reversed. They gotta look weird for the stupid people to remember. Offer incentives so these cars are $800~$1200 lower new with tax credits. Slowly people will start buying new cars with the "weird" clusters.
3. After 8 years, new federal signage only has KPH. Keep the units. It will still take over two decades at standard road maintenance rates to phase out all the old signs entirely.
4. New generation accepts KPH, occasionally takes photos of the old vintage signs, next generation of YouBoobers (or whatever succeeded Google/Rumble) posts videos of restoring older vehicles with EU gauge clusters so they have KPH.