When the year is over you MAY file a tax declaration if you had 1 or 4/4 combo - or you MUST file it if you had 3/5 or 5/3 combo (because then you have normally underpayed the taxes through the year as a couple). If you have 1 or 4/4 you have normally overpayed taxes so if you don't file, the goverment is happy.
The tax code in Germany is mindbloggingly complex. And it's not just laws, its' also "common practices", knowing what the Tax Authority (Finanzamt) accepts and what not. It's also knowing the current lawsuit which may be potentially applicable to your declaration. Every year there's a few dozen such tax lawsuits where the results are applicable to large groups of taxpayers.
When I was single I either didn't file the tax declaration or filed it myself with the help of software and a few books. There's a bestselling book "1000 absolutely legal tax tricks" (https://www.amazon.de/Konz-1000-ganz-legale-Steuertricks/dp/...) which is a a good starter.
Nowadays with family and kids we must file a tax declaration, but now there are so many special cases we're either subject to or can profit from that it's not realistic to do it on our own. We outsource our tax declaration, it costs us around 300€ a year and is absolutely worth it. Our tax consultant routinely argues with the Finanzamt over two-digit sums, basically fights for every cent.
Honestly I don't think we'd better off if the tax system in Germany would have been simpler. We profit from a number of special cases like "extraorinary burden" because of the disabled child, so I think in the "one size fits all" system we'd probably lose. I'm totally fine with 300€ fee for the tax consultant, she allows us to use the benefits of the system. By the ways, this fee is considered the next year, we get out tax back from the fee.
If I remember correctly, a few years ago politicians discussed a "no-file" option: you declare that you don't file you tax declaration this year and get some bonus back (I think the proposed sum was around €300). The Finanzamt has less work to process declarations, so probably this was worth it. I don't think it was implemented, however.
Get dozens of book recommendations delivered straight to your inbox every Thursday.