The people that were societally oppressed in the USA during the middle class boom were in their bad situation due to other societal ills, not the taxation structure.

I'm not saying that the entirety of US policy was good then. Clearly there were many societal ills, including widespread gender and racial discrimination in housing and hiring, terrible literacy rates and targeted violence against ethnic minorities in the rural south that persist to this day, and religious bigotry was widely accepted. The economic structure, though, successfully allowed for personal wealth while limiting it, and created an undeniably huge middle class. The fact that many citizens didn't get to participate in it was due to those other non-economic social problems freezing them out.

Also, mid-20th century USA is a single example of a system that was brought up to illustrate the point that there were more than the false dichotomy of choices presented. Surely there are way more ideas out there than status quo or status quo + UBI.

UBI has no precedent for working, and I, some rando online, have already identified a potentially disastrous problem that undermines it that I've never heard any convincing solutions for.

I love gaming out problems and solutions, but it is important not to fall in love with our ideas. Getting upset when holes are poked in them or ignoring these weaknesses aren't going to prevent our opponents from exploiting them. If a plan has intractable problems, there is no shame in making new plans that may avoid those problems.