Cheapest is peer to peer. Find someone who wants to exchange something for btc. I'll exchange $ for btc at a 10% discount.
so how do you avoid the high eth when sending from metamask to a exchange, why is it preferred to swop first before sending crypto to the exchange and simply trade it to usdt there?
Just sell on centralized exchanges Coinbase Pro (Advanced Trader), Kraken or Gemini The fees can be high at times on the Bitcoin or Ethereum network, but as long as it's only a transfer, wait for when the network is not too busy, it will be less than $3 tx fee