Taxing investment income is complicated

These are good options when available. You should start by setting all the Pigouvian taxes at optimal levels then go from there. Having not thought about it very much, taxes on unimproved value of land seem good but can't fund something like a modern government without major distortions, so you'll end up with lots of other stuff in your basket.

I don't think any taxes have zero social costs. Maybe you're imagining that they have a net zero cost, i.e. where costs born by some people are offset by gains enjoyed by others?

It's maybe off-topic, but I'm concerned by the accounting realities attendant to some of the taxes you mentioned, and similar ones, e.g. carbon taxes and cigarette taxes. It doesn't seem likely that either would or are actually internalizing externalities in practice. Cigarette taxes are often 'earmarked' or allocated to entirely unrelated goods or services, e.g. schools, and that can be disastrous when smokers actually respond to higher taxes by buying fewer (legal, i.e. taxed) cigarettes. Similarly, it doesn't seem like the costs of 'carbon' are actually internalized by the tax itself if the tax revenues themselves aren't directly used to offset those costs by, e.g. capturing carbon, reimbursing losses incurred because of 'carbon', or, more sensibly, saving to offset the expected future costs.

https://en.wikipedia.org/wiki/Deadweight_loss

If this is true, wouldn't we have to worry about correlation between types of tax? Taxing A at 1 and B at 1 has social cost 2 if they're totally independent and social cost 4 if they're actually the same thing.

To be clear, this is the low-order approximation around 0; as explained in Paul's link (sibling to this) the effect away from zero involves the shape of the supply and demand curves through the relevant region of prices (and the stated claim holds when they're linear).

The solution you propose also sounds really complicated when people have to optimize the timing of when they make capital gains with times when the risk-free rate is low.

When you sell assets you deduct the amount you paid for them. The proposal is to multiply that basis by the total amount of risk-free interest that would have accumulated over the intervening period, which can be calculated by looking up a single number in a table. I agree that using the risk-free rate when you sell would be insane.

(From the perspective of tax optimization, I think this is much simpler than the status quo. From the perspective of tax accounting, this mechanism takes the place of the distinction between long-term and short-term capital gains, and is radically simpler than that.)

Also, my current understanding is that risk compensation is definitely not the large majority of investment returns. The last chapter of Cochrane's Asset Pricing text has a great discussion of the topic. The main conclusion is that explaining returns via risk exposure requires unrealistically high levels of risk aversion - like, one or two orders of magnitude above the risk aversion levels implied by other activities.

What's the competing explanation?

Haven't looked at the historical numbers, but in recent times it seems like (i) with log utility and a naive model of "future=past," optimal leverage is around 2x, (ii) most investors are much more risk averse than log utility (even for casino risk). So it seems like things basically add up here for most of the market. Was the situation an order of magnitude different in the past?

This is basically the argument in my second bullet. You can make what you will of that argument, I think I still believe it---taxing savings just seems strictly worse than a similarly-progressive tax on income.

(I also don't much like a wealth tax for the same reason.)