General question from a foreigner who has lived in Germany, does it seem to you like those income boundaries for benefits are designed to discourage class mobility? Do most benefits seem designed with hard on/off boundaries, or are most designed with a gradual decline as income increases?

In my very limited experience talking with Germans, it always seemed like those social benefits were a boolean choice that kept people from wanting to move up.

That could have just been my interpretation of the culture though, where it seemed many Germans simply seem to accept that retiring as a cashier is an acceptable level of ambition. That level of ambition is definitely not the same world-wide, and can come off as "lazy" to non-Germans who may not understand what it's like to simply accept one's lot in life.

Benefits often have some fuzzy borders (e.g. 450-Euro-Jobs/Mini-Jobs), but will quickly lead towards "make one Euro, lose on Euro of benefits".

As for ambition, it may have to do with a lower income disparity, where a cashier in a grocery store will certainly not make great money, but will be able to support themselves (median income in Germany is ~2500 Euros, full-time cashiers will make ~2000-2200 Euros). It's also often not a full-time job, but 20-25 hours per week.

Sounds like the welfare system in the US. Same obvious result, obviously. Fixing that would be a concrete step towards UBI.

As far as I understand, most UBI-proposals would essentially be the same. If you work, you pay more taxes on your income to fund the UBI. You'll still get the UBI, but you'll pay significantly more taxes which will leave you pretty much even.

No. The whole point of UBI is to remove the cliffs and high marginal tax rates that the working poor face; people in low-income jobs would benefit the most, approximately doubling their income (someone on minimum wage would instead get the equivalent of minimum wage + unemployment). Also it becomes more practical to ease into work, since if you work e.g. 5 hours/week you get full pay for those 5 hours rather than losing more in lost benefits than you gain. Of course this has to be balanced by higher earners paying more tax.

We have some of this already included. If you work in so called Mini-Jobs (up to 450€/month, employers do not need to pay into social security, reduced bureaucracy and public health insurance premiums), 100€ won't change your benefits and 20% of whatever more you make will also not affect your benefits. This is primarily meant to be exactly that: low-threshold ways to work for a few hours and have extra benefits.

Imho the primary issue remains: it's not required to work, you won't get a massive improvement if you're okay with the current situation and you have to expend energy and do stuff you don't love. Open source projects that rely on donations often face similar issues. Donations are not mandatory, so few people will donate, as they can still use the software without contributing. I don't believe that "donating is difficult" is the issue there either, most accept paypal, github sponsors etc.

As a counter example: a UBI proposal in my country was linked to taxation on consumption (VAT). That makes more sense at first glance.

But the cutoff happens much later in the curve. With classic welfare handouts the marginal tax curve is very steep at the beginning