My point, again: it’s not required that you reach for a grid framework to build a grid. You can do it! This is also not to say that flexbox is trivially easy and you’ll never have any problems. There are plenty of edge cases and weird browser support things that you may run into if you start using more niche flexbox features. The stuff we looked at here is pretty vanilla though and I’d be surprised if you had problems.
Some good flexbox grid advice from Chris Coyier. Chris has a knack of breaking things down and keeping things simple, stupid.