So, for various reasons, you'll want to cash out (as fiat) some of your BTC savings and the questions would be: how much one will pay for fees per such transaction?
Before answering this question, can you tell how one can calculate the fee for buying BTC? I see the fee is calculated based on the transaction data (size) to be processed -> how to calculate that and minimize the fee cost?
Once you bought BTC, what would be the fee for sending this further to another wallet (let's say a storage wallet)?
Definitely not custodial wallets. Most custodial wallets can benefit from imposing higher than appropriate fees when their users withdraw and the transaction is a batched transaction, which means that you're really paying more than what's needed/spent on the fees.
As for the wallets to use, you can choose from Electrum, Wasabi wallet, Bitcoin Core, they're my recommendation. For the lowest fees, try to use Segwit. The wallet will calculate the size and thus the fees automatically for you. Since the size is a direct factor of the amount of fees that you pay, try to reduce the number of inputs used per transaction by consolidating smaller UTXOs periodically when the fees are low. For example, if you receive 3 payments a day, you can consolidate those many UTXOs and you'll need to use lesser inputs when you need to spend your funds in the future. The benefits is most evident when you're trying to send funds when the fees are high.
If you want a rough estimate of the size, try using this:
https://jlopp.github.io/bitcoin-transaction-size-calculator/. You have to understand what the values means. You don't have to know the size, just follow my recommendations and you'll be fine.
I can't comment on how exchanges charge for the fees, that's dependent on where you live and which exchange you have access to.
