"Is there public evidence of billionaires giving half their income to HMRC"
No, but there again, neither do estates that are worth £500k or £1 million
From the HMRC data (which covers 2021-22) it shows that the largest estates, with an average net value of £19.5 million paid on average £3.9 million in IHT.
That is 20% of the total value. So, quite a bit less than the roughly 40% you would expect. So it shows that, yes, they are using means to pay less IHT but they're still paying a lot. (You would typically expect an estate worth £19.5 million to pay £7.4 million in IHT, so they're saving about half of it)
Moving down to those with estates between £2 million and £10 million they ended up paying on average 24% of the total value of their estate in IHT.
If you take a person with an estate worth £5 million then you would expect IHT to be typically either £1.6 million or £1.8 million. These people paid on average £1.46 million in IHT. So they were managing to save maybe a couple of hundred thousand.
Then move down to those estates worth £1 to £1.5 million. Here, around 23% of people didn't pay any IHT at all so they obviously managed to do something.
Of those that did pay tax, the average amount of the total was 13% or £155,000.
You would expect an estate worth, say, £1.2 million to pay either £80k or £280k in IHT so it looks like it was a mixture of single people and married people who were paying the IHT.
Then if you go down to estates worth £500-600k then only 19% of estates had to pay IHT at all, which would imply that for a lot of these people they are getting the £1 million allowance.
Of those that did pay IHT they paid on average 10% or £53k in tax
So, yes, this tax is hitting the "struggling middle class" the most - if your definition of that is somebody who leaves an estate worth around £5 million.
Source: HMRC Inheritance Tax liabilities statistics Table 12.1
https://www.gov.uk/government/statistics/inheritance-tax-liabilities-statistics