A $20k American-made electric pickup with no paint, no stereo, no screen

Not clear what is meant here. Does ethanol from corn count? Methane from waste dumps? Gray hydrogen? Wood pellets? Ammonia?

Electricity from unclear source?

Human ingenuity is infinite. It is not enough to enact simple rules, people will just produce electricity with hydrogen and claim it green if it will make them profit. If it will help them evade carbon tax. Nevermind that hydrogen came from some extremely polluting process involving damaging our planet atmosphere and everyone's health.

Well, you don't need to tax the ethanol from corn or methane from waste dumps or wood pellets, or ammonia itself. You would tax the oil/gas/coal that came out of the ground that was used to fertilize the corn, process the corn, transport the corn, and distill the ethanol (otherwise it's double taxation). You don't need to tax the wood pellets or the stove they're burned in, or the electricity, just the carbon that is burned to make and transport them. So this is largely irrelevant.

A better question would be for imported items and services. How do you prevent tax shifting from carbon emission havens, which is no different from financial tax havens now. You tax them at entry using the most beautiful word, "tariffs". If an importing country doesn't tax carbon or carbon tariff their imports then you tariff them. Interestingly, it would then be a higher tariff for air transport than shipping. Where it actually get complicated is services, which people really don't like taxing. But if I run a LLM datacenter on coal in china or make bitcoin burning middle east oil, or consult on green projects on Indonesian gas those should be tariffed as well, and that's more difficult.

GP said it very clearly: tax the fuel. Taxing carbon atoms as they come from the ground is very different tax.

It is way harder because of reasons you mentioned. It also disincentives, e.g. composite materials with carbon in them. The materials of today and tomorrow, which would make us all richer and planet healthier, because of how much stronger carbon-nanotube reinforced concrete is.

It will disincentive fertiliser production, which can be net carbon-neutral or better.

It will disincentive carbon capture at power plants, since the tax was already paid. Graphite production.

This is my point: every policy is necessary complex. There is no "simple" and "just ..." when making policy