Is this the correct way to calculate security for bitcoin proof of work?
Not sure. But looks like lot of things are ignored in these numbers. I have read about FRM so I will share few things related to FRM ratio and security:
The Fee Ratio Multiple (FRM) is defined as the ratio between the total miner revenue (blocks rewards + transaction fees) and transaction fees. FRM is a measure of a blockchain's security and gives an assessment how secure a chain is once block rewards disappear. This metric was first introduced by Matteo Leibowitz.
A low FRM suggests that an asset can maintain its current security budget (miner revenue) without having to rely on an inflationary subsidy.
Conversely, a high FRM suggests that an asset will require heavy inflation via block reward subsidies in order to maintain its existing security budget.
If a government or group of few rich people or organizations create a token, hold most of the tokens and stake it, price of this token and few people holding this token makes the system secure?
No. I don't think this makes a system more secure.
What are other differences in proof of work and proof of stake?
Few differences are mentioned in the answers for this question: Why doesn't Bitcoin migrate to proof-of-stake?
"Proof-of-stake is still in its infancy, and less battle-tested, compared to proof-of-work"
Although difficult to quote anything from websites associated with altcoins, I agree with above sentence which is from https://ethereum.org/en/developers/docs/consensus-mechanisms/pos/