The whole point of guaranteed minimum income is to make participation in the job market optional: workers are free to sit out the market if employers try to give them a raw deal.
For instance, Milton Friedman noted that it could be rational for a worker to voluntarily work in a dangerous environment and trade his long-term health for short term money, because his health in five years doesn't mean whole lot if he starves today.
The government's response to this is OSHA, a bureaucracy designed to publish and enforce minimum standards for workplace safety.
But with guaranteed minimum income, nobody's going to starve, so employers will have to offer a safe work environment to entice workers into employment. No bureaucracy is needed to protect workers in desperate situations because there aren't any, everybody can make their own decision about how much risk they're willing to accept to work a given job knowing that their basic needs are provided for.
It IS possible to get spending in check, and fix everything, but we need to get rid of a LOT of bloat, there are WAY too many uneeded jobs in congress(especially in congress/and the senate lol). Personally, if I had a gbi, I would NOT work--in the basic sense, I WOULD innovate and do freelance work to earn extra money to put towards a startup idea... That is what I love about GBI -- the fact that entrepreneurs don't need to work but can actually work full-time on their business ideas!
A national sales tax comes with its own set of bureaucracy. It's also regressive and shifts the tax burden disproportionately onto the poor (unless you made the sales tax wealth or income dependent, which would introduce the same bureaucracy you were trying to get rid of earlier).
I'm not sure a GBI would necessarily be good for entrepreneurship. I see your point, but having a runway disciplines young companies and forces them to get to market in a timely manner.