I know you meant this sarcastically, but yes, flex is a good option for centering something. Either that or setting the left and right margins of the element to auto, which is generally even easier.
Basically, if you’re in a flex container use flex, if you’re in a grid use grid, and if neither of those apply set the left and right margins to auto.
Well, flexbox and grid have different purposes in my opinion/experience. Personally I use grid for “top level” layouts like the layout of the whole site, while I tend to prefer flexbox for layouts inside the grid. Of course that’s just a rule of thumb, there are absolutely cases where this isn’t the best option.