Are there any low-cost bond index funds available? Are there any unit trust alternatives similar to A35 ETF offered by nikko am?

There are cases to be made for actively managed bond funds.

https://www.investors.com/etfs-and-funds/etfs/b...
https://www.google.com/amp/s/www.nytimes.com/20...

The bond market is dominated by pension funds, government, many which are passive investors who are more concerned about getting certain exposure (geographical, or credit rating), rather than pricing risk-reward efficiently. This makes the case for certain active bond managers to be able to charge a higher fee, but still manage to generate enough alpha to justify their value.

Try to avoid unit trusts

you could think of physical gold as a bond substitute

some experts fear that the next big financial crisis will happen within the realm of corporate bonds

more on my thinking:

https://seedly.sg/questions/what-is-your-genera...