"Ultimately a UBI is just the tax money given back to the people it was originally taken from."

Well.... no. Because it is going to be a tax on future-you, not past-you. The past "taxation" is gone. It has been spent. It isn't there to give back. Anything government "gives" you it has to either steal from someone else today or counterfeit through the Federal Reserve (or the equivalent).

"Taxation" is theft. Nothing can change that. Yes, theft can benefit some people at the expense of others. Some thieves even put the stolen money to good use-- benefitting widows and orphans. It is still wrong to take property which was honestly gained through work or trade, which the rightful owner would rather not lose. It doesn't matter what you call it or how you justify it. Or how you spend it.

"My biggest argument for having a UBI is that the people who work for Social Services in the government hate the idea. They would become redundant if the welfare system was eliminated."

I guarantee you they'd find another tax-junkie position. People who go into government "jobs" are rarely fit for anything honest or productive.