Is it permissible to break the fast during the day in Ramadan due to the effect of fasting on work performance, which could lead to the loss of the job and consequently harm one's livelihood, especially given that the fasting period reaches 18 hours daily?

It is not permissible to break the fast initially. Look for a night job or take a vacation in Ramadan. If that is not possible, then look for another job that allows you to combine fasting and working. If all of that is not possible, then intend to fast, and if fasting becomes extremely difficult for you, leading to loss of concentration and disruption of work, then break your fast and make up for that day.