I don't understand what you're saying. I agree that food insecurity could be fixed; it's a redistribution problem. UBI is a way to fix it.

But I don't see how we are forcing people to be low class with UBI (as opposed to having them unemployed or doing precarious work that doesn't pay even the basic needs).

The problem with UBI is that it's presuming the economic system mostly remains as it is (supply and demand -> buying and selling), but the suppliers will merely keep increasing their prices to maintain status quo. Unless costs are also heavily regulated, which is tangential to the implementation of UBI. In the end the only thing UBI does is delay the inevitable crash of the economic system. Maybe.

How much UBI would be needed for most of the population to live comfortable middle class lives? UBI isn't going to be acceptable if it reduces the middle class to just above the poverty line. People want more than just having enough to eat.

So while it may solve the problem for people who are food insecure, it will create a problem for those who were better off with good paying jobs. You can't just reduce 95%+ of the population to simply meeting basic needs and not facing serious societal discontent.