Robot are just means of production, capital. We don't need to invent robot taxes; we just need to increase taxes on those people/companies with the most capital and the most benefits (well, and close loopholes).
The question about approaching full automation is whether all that wealth will be for humanity or for some selected rich people.
It might be easier to tax physical bots though than the capital behind them. Just like with individuals as opposed to anonymous companies. Zero's and ones are easier to push around.
Capital in the economic sense is not just dollars, it includes buildings and trucks and cranes and agricultural tractors.
The minute you overspecify a law to apply only to some products you are creating opportunities for loopholes. See this classic story about vans vs cars, and import taxes:
https://www.bbc.com/news/business-45875405
Is a John Deere tractor a robot? What about a roomba? A 3d printer? What about a disassembled robot? The wheels of a robot? How do you define robot?
What do you tax, the purchase of a robot? The use? The ownership?
There are very tricky questions that try to fix an artificial problem. Tax the company or the person behind x, be x a robot, a field, or a brand that generates money (or not, if you are against taxes). This is not a new problem.
I agree about the principle you propose. I'm certainly not saying taxing the robots is how it should be, just that it might be easier.
Governments might rather tax robots than properly deal with anonymous corporations playing extremely aggressive tax avoidance games.
Pushing money around the world has become so much easier. Try to make countries give up on fiscal anonymity, tax avoidance marketing or even plain tax evasion marketing. It might work on some micro nations, but good luck trying it with countries like the US...
The question about approaching full automation is whether all that wealth will be for humanity or for some selected rich people.