Capitalism is already a caste system. UBI would just ensure the lowest rungs don't starve or lack housing/healthcare/other essentials.
Not sure how you're connecting "UBI → caste system," so you'll need to show your work on that one.
If we can fix our government so that it actually works for the people instead of the corporations, then I'd say we should nationalize every publicly-traded company, install price controls, use the revenue to fund UBI, civic infrastructure, and social assistance programs; and then balance the sheets so that each company barely breaks even without extracting any profits. That can all be done without violence.
Your "violent nationalization" sounds poorly thought out. How do you go from "violence → nationalization"? I don't really see a logical progression from A to B.
And what about C, as in, what comes after that? Okay, you nationalized the companies. Now what, who controls it? The government? What happens if people keep electing corrupt officials? And what benefit will nationalization do if you're against UBI anyway?
And that's all without even touching on the vagueness of "any company with more than a handful of employees." How do you define that? An LLC with 6 or more employees? That could include most mom&pop shops. So you just want to murder old people and take their livelihoods, entrust it to the state, and what, profit?