This sounds great in theory, and I’m sure it works great during a bull market for the ages.
Following this “strategy” when the bottom drops out of the market will get you into serious trouble.
Also, when inflation picks up, interest rates will rise, and the 0% into offers for over a year will dry up.
Bottom line, if you do this, you’re skating across a deep river on thin ice, saving the time and effort of finding a bridge, hoping you can make it to the other side before the ice breaks so you don’t drown.
However, as the saying goes, “Hope is not a strategy.”
While not as rosy-sounding, you can safely use part of this method, the cash back (and incentive bonuses), and instead of trying to invest the money at risk in the market, use safe assets such as a high interest savings account.
The returns (including the cash rewards) will not be 12%, more like 3%, but you don’t risk bankruptcy.
Of course, you still need to be reasonably frugal and super disciplined to make any of this work.