Should I do voluntary contribution to my CPF Special Account to hit the Full Retirement Sum as soon as possible or to invest elsewhere with the cash on hand?

Regardless whether you have better investment option. it is much safer to hit FRS and you don't need to think about it anymore for the rest of your life. By CPF SA alone, you will have a comfortable retirement if you do it early enough.

I do know people do voluntary contribution to CPF MA, then CPF SA, to have tax relief. I do think if your annual salary is very high, then it can be worthy to contribute to CPF to reduce tax liability.

I did voluntary contribution for a few years.

You cannot reverse cash that is transferred into cpfsa. Better to invest.