Construction costs are currently skyrocketing

Well, that's exactly not the case. On the contrary, it lowered your tax burden. The alternative, where you only pay taxes on your own income (=individual assessment), could have been freely chosen by you but would have cost you more taxes. So I'll turn the tables: the community financed your tax advantage ;)

I am denied joint assessment due to lack of marriage, which costs me nearly a five-digit amount every year.

No, you can't see it that way. It has indeed increased our tax burden because it raised our tax rate. He also earned money in Germany that year, which was of course taxed in Germany. But on both our incomes in Germany, we were then assigned a higher tax rate because of money he earned and taxed (and spent ;) ) as unmarried in Australia in the first half of the year.

Well, exactly that did not happen. On the contrary, it lowered your tax burden. The alternative, you only pay your taxes on your income (= individual assessment), you could have freely chosen, but it would have cost you more taxes. So I turn the tables, the community financed your tax advantage ;)

I am denied a joint assessment due to a lack of marriage, which costs me nearly a five-digit amount every year.

I first doubt that income taxed abroad still has to be taxed here in Germany. The progression clause works differently. I suspect the tax return was prepared by yourself and not by a professional...

You are by no means denied joint assessment? Who denies you marriage?

is not saying that the income was taxed again. But similar to sickness benefits, maternity pay, etc., it was counted as existing income and increased the progression rate.

However, .. your income from the first half of the year was then also for two people and you paid less tax on that portion or presumably, due to the increase in progression, the tax rate you would have paid in the first half of the year as a single person.

I agree.
At the other end of the tax-sense spectrum, I always found the progression clause. When my husband came to Germany, he had to declare income he earned and taxed in Australia again here. It wasn’t taxed directly, but it increased our tax rate... o_O

I find especially the progression clause on parental allowance or short-time work allowance completely nonsensical. That is already calculated based on income, there’s no need to levy the tax again indirectly.

No tax is levied on it either; rather, this income is calculated as if it had been gross income in that year, and it is virtually fictitiously added to the rest of the gross income of the respective year. This results in the applicable tax rate, which is then calculated backwards.

Example (no exact figures, just illustrative numbers):
net paid sick pay: 7000
remaining gross income in the year: 44000

7000 is then recalculated to 8500 gross
==> 52500 gross ==> 9000 euros tax, which is then divided again in the ratio 44000/8500. The share on 44000 is the tax liability for the year. And the amount is in this way somewhat higher than if the 44000 euros had been considered the entire annual income.

No, no tax is levied on it. But a higher tax rate also means more taxes.