Bitcoin Core – How to determine custom network fees?

I’m a bit stuck. I haven’t bought BTC since October 2023 and didn’t realize the network fees were so high, so now I have some BTC on the exchange and have to pay around £20 to send it to one of the following: I’ve been keeping my wallet there hoping the fees will go down.

But it got me thinking. How can I determine the appropriate custom network fee amount using the BTC Core wallet? They also assume that if the tariff is too low, they will eventually go back to the source because no one will check. Is that true?

I would like to know how to determine what a reasonable network fee is.