Just move your bitcoins to a CEX that is regulated & legal in your country and convert them into fiat from there. CEXs that are affiliated with the government will never have problems with taxation, because every transaction you make is counted in tax, that's what happens in my country.
Also, make your transactions staggered not one time big time transaction. If you can make it on a daily/weekly basis, much better. I have had the same thoughts before and what I did was I converted my btc to fiat at different time intervals, not on a daily basis though. Use exchange platform that is reputable, and if you will use your local exchange, better make sure you submitted legit docs. Don't give them reason to freeze your account for some carelessness.