The fun part about advances is that in the first year they don't have data to set your advances so you have to back pay the taxes for the first year in full after filing the first year's taxes in the second year in addition to now having to pay the advances on that year so even if you saved all the money you expected to have to pay in taxes during the first year you also need to have saved up enough to pay the advances but the more money you made (allowing you to set more money aside), the higher the advances will be.

You can negotiate to have your advances reduced but the government really doesn't like it when businesses (including freelancers and solo entrepreneurs) have a high difference between their advances and the actual taxes. This also goes for declaring your VAT (which depending on how much you make you'll have to do monthly or quarterly as an advance and then alongside your income tax) and while in that case you basically set the advance yourself you'll get in trouble if it doesn't match your actual VAT (difference between charged and paid).

You can file your own taxes and if you're a regular employee there are tons of non-commercial orgs you can go to that will assist you in doing that but if you're a "business" (even if it's just you and you haven't incorporated) they'll often not help you.

BTW the reason you can just forego filing your own taxes as an employee if you just want to be taxed on your wages (and don't have anything else to declare, e.g. interest on savings) is that your employer already had to do the taxes on your wages and benefits for you. So it's not like the government just does them themselves, it's just done by payroll instead of you.