Yume97 From the images you've posted, your PC interface is different from mine (US versus Europe???). So I can't give you exact answers.
You should not pay for any more coins until you have used the all of the coins you have. So do not use top up.
When I go to webnovel.com/bill/coin (or click on the coin icon in my avatar/profile), it tells me how many coins I have, and then tells me how many bonus coins (coins that expire in less than a year) are included in that number. Underneath that, it says what coins will expire first and when they expire.
Once you have used enough coins so that all of your coins are bonus coins, then as long as you are reading some chapters every day, you should not lose any more coins due to expiration. You can just keep on reading chapters like normal until you run out of coins. After that you can use top up and pay for some more coins and you'll be fine for at least a year.