If we want to turn altcoin to cash, will we need to sell it for BTC first, and then send the BTC to service for conversion to USD/EURO before finally to bank?
That is definitely the wrong decision. because the fees for processing Bitcoin blocks are very high. You will have to pay 10 times more money than dealing with the top alts.
You can trade by LTC, XRP, ETH. Transaction costs only fluctuate around $ 0.04 - $ 0.01, which is 10 times cheaper than Bitcoin.