God help me, somewhere around 2016 I started using some alpha version of Bootstrap 4, and I just keep using that same version for all the basic stuff underneath my own SCSS for whatever I'm building. It's absolutely fine. A lot of it is displaced now by grids, but for flexbox stuff and modals and pushing rows and columns it just works. Very easy to remember class names and extend it. Thankfully I don't work for a company that cares about using the latest thing.

The only really critical things I need in scss are scaling typography and variables representing colors for iteratively generating class names and gradients and transparencies and stuff like that. By "scaling typography" I mean, I just have a loop that writes .scaling-typography-[a]-[b] where a and b range from 1 to 16, and the class that's generated scales from [a]em to [b]em between a phone size and a desktop size. In a rare case where that doesn't just work or it doesn't look good on a tablet or something, a couple specific rules extending xs- or sm- or whatever will cover it.