Newbie question on selling Ethereum and fees

I'm using Coinbase and have around $1000 in my wallet. So far I'm just holding and haven't really transferred anything out yet. I'm also not using leveraging at all and have a principle of only buying money I can afford to loose.

But when I do want to sell, are the transaction fees directly tied to the amount I'm transferring? Or is it a set fee?

I also wonder how I should sell and into which currency. A friend of mine told me it would be wise to take the profit by converting it into a stable currency, like USDC Stablecoin.

Is this the general idea? I have read horror stories on tranfer fees and since the amount I have so far is so little, I don't want the profit to be eaten by transfer fees.