Fisicx is right you should look at mobile-first as an ever-increasing number of users are view via mobile and tablet first on websites. I see far too many sites looking the same due to bootstrap, think about what the users need to do on the site and how they will get around, what they need to see.Depends on your users:
Can I use... Support tables for HTML5, CSS3, etc
caniuse.com
Don't try and build the whole site using grid. Use it for the components that need to be in a grid. If you are developing using a 'mobile first' approach you shouldn't need much more than a vertical layout. Bin the sidebars and horizontal rows, they are a things of the past along with bootstrap.
Floats and media queries are still very powerful, don't overlook them in creating layouts.
Looks good, flexbox can be tricky to work with at times but it's useable, https://www.w3schools.com/css/css3_flexbox.aspI've started using Bulma as my framework, I never used Bootstrap so I can't compare. It uses flexbox for layout and is pretty easy to add/remove the elements and components you need. It's not 100% perfect and I still find myself writing custom CSS but I think that's the case with any framework.
One of the problem with using flexbox (and frameworks in general) is the DOM. You often end up with excessive DOM elements (something Google doesn't like) and stacked CSS classes.
Yeah that's one of the annoyances I have, it's often div inside divs to get the desired effect. I'm mainly using it because it's quick to work with, which for what I'm working on right now is fine. Being able to tweak it through Sass variables is handy, though there's still a few bits that are lacking. It's also actively maintained. I've tried a fair few frameworks for layouts which start off strong but quickly fall behind on updates and become defunct. Any recommendations for a framework? I mainly just want layouts, columns, etc. I prefer to style all other elements myself as needed but having defaults I can change would be handy.
Have you ever used W3.css by W3schools, I have used it a couple of times and found it quick and easy to use https://www.w3schools.com/w3css/default.asp
I haven't, W3Schools used to be sh*t and full of misinformation. I think it's improved in recent times, but I'd still be reluctant to use anything they created in production. I'll check it out though.