You should only have to do that in the beginning and strive to earn more so that you can hire an accountant or a firm to keep the booking and calculate your taxes for you. Invoicing is a matter of minutes and some apps even do it for free. Once you've set up your templates for each type of contract and document, you should be good to go. The only true problem in freelancing is generating new (good) leads. That's it. Admin stuff is a piece of cake with today's technology.