Can't get away from Flexbox entirely. At this date, I found that Flexbox was easiest (or simplest) to work with when working on utilities. While utilities often involve a bit of layout work, the bulk of the task is a utility. One example is a complex responsive navigation system. If it is a simple responsive navigation system, there is absolutely no question about it: use CSS Grid. It also depends on how "complex" you want to get.
I'll give you an example. My navbar has three sections. Brand (logo), Links, and Admin (or Search and/or Burger) area. When in Mobile mode, the Burger displays. Click it and the mobile menu slides in from the left. The mobile menu has a title area displaying the currently selected main menu ... it displays when you click on a link with a submenu ... submenu displays, title bar shows the main link and a collapse arrow.
Just by that description, you know that it needs Javascript to make that complex responsive navbar work. No way that CSS can handle that.
As soon as you involve drop down menus, use Flexbox. The difference is that it is easier to control a Flexbox where you are adding and removing columns, than do that in CSS Grid.
I'm still stuck, however, in finishing my CSS Grid file. By my reckoning, Flexbox is not a good choice for a responsive layout system. And that is not taking it lightly ... it's an opinion at this point of time and it has taken some time to get here. I've taken some previously finished projects and used them as development tools to evaluate CSS Grid. So far, I've found that not only do you save a lot of time with much fewer CSS statements, you also save a lot of time with fewer HTML tags and attributes. CSS Grid presents as a far more intuitive tool. It's easier to read, easier to follow, and easier to trace issues.
Where it is a tad more complicated, and that might be because I am thinking through the process, is in "hiding" columns. In other frameworks, it usually manifests as a "col-0" class that has a "display:none;" definition. It's not quite that simple in CSS Grid. At least, at this point in my research.