This is effectively false for the vast majority of tax payers. Tax preparation services can still exist for the relatively small number of people that require it.
If you are imagining W2 workers with no business activity, children, inheritance, donations, property, or investments, I think this is actually a tiny portion.
Heck, nearly 90% of households take the standard deduction.
The IRS should send a pre-filled form with an amount owed (in either direction). If you agree, you sign it, and make any payment/refund. If you disagree, you add your deductions and whatever else needed, similar to today.
Instead they could just send you "This is what we calculated" and you can do nothing (probably 90% of people) or file as we do today for the special cases you mentioned.
Please explain. Which percentage of the year did the child live at each household? Is the property your primary residence? Did you rent a room? What amount did you spend on property upkeep or interest?
[0] https://neotax.eu/en/blog/tax-classes-steuerklassen-in-germa...