Do I need an agency to manage my Google Ads account? No, you can do it yourself. So there are two things you need to look at. First of all, have you got the time and the energy because it's not a case of set and forget and let it run. Yes, Google is going to run and spend all your money, whatever you're throwing at it, but you may not get the best results, so you need to weigh up the options, where you get an in-house person to manage your campaigns and budgets, if you are spending a significant amount of money, or even if you're spending more money, and you haven't got the time then you most probably throwing money and wasting money on Google or Facebook or wherever.
So you got the option you either do it yourself if you got the time. Or if you don't have the cash to hire somebody then you got to do it yourself. You need to learn Google Ads properly. That will require investing time. What would you rather do? Focus your energy on running your business or trying to learn digital marketing and Google Ads. The third thing If you can't do it yourself, hire somebody, a freelancer, part-timer or an agency or an external person to manage your accounts. Reply to your question, no you don't need to do this through an agency, you can run it yourself, I have lots of students who are successful business people and they manage their own accounts.
If your account is really well organized you don't need to spend hours on end. And I'm talking about small budgets between $5000 to $10000 a month. You can easily optimize a campaign or an account. Between 15 to 20 minutes a day. In the morning and maybe 5, 10 minutes in the evening just to keep an eye on things how it's doing. But other than that you don't need to be sitting in the account and making changes because you harm your account and your campaigns if you make too many changes because you keep slamming the breaks every two minutes and making changes. Let it run once you made some changes.