Not OP, but I recommend using plain css. With the grid layout and flexbox it has become pretty easy to create something decent and learning flexbox and grid isn't hard. Grid works basically the same as Bootstrap: split a page in parts and tell each element how many parts they can use. But with gird you don't to split your page in 12 part, but can choose for yourself how many you want. Flexbox is rid, but for things you want to arrange 1d instead of 2d.

However, if you know how to work with Bootstrap, I don't see a reason to learn something else for side projects, if you want to minimize the effort you put into front end stuff. It might not be ideal, but it's good enough imo.