Virtually every website I’ve built has used PHP, so to me, an editor with a debugger is indispensable. Not having a debugger is a deal-breaker not matter what else the editor does. My favor editor was Notepad++ with the XDebug plugin. Unfortunately, the plugin is no longer maintained, and it got less and less stable with each new release of Notepad++, so I finally had to say goodbye to those two.
Now I use Codelobster, which doesn’t debug as flexibly as the Notepad++ plugin did, but it’s the best free, non-Java editor I’ve found (that has a debugger built in). If you don’t mind Java apps, there’s Netbeans, which as a debugger and a whole lot of project-oriented features.
I’ve never liked site builders that feature drag-and-drop components. Like WYSIWG editors, they hide the code and you don’t know what’s going on under the hood.
I do like some Content Management Systems (CMS), like WordPress, but my favorite, especially for smallish sites, is Wolf CMS. It’s like a page-stamping machine. You set up the template, as complex or as simple as you want it, and you enter the content separately. Unlike WordPress, it doesn’t impose a blog orientation you have to escape from with plugins. Unlike WordPress, it’s also very lightweight, which is why it’s especially good for smallish sites.
You mentioned Bootstrap. Bootstrap is a framework that includes tools, templates, etc. for HTML, CSS, and Javascript (not PHP, as far as I know). By all means, download it and check it out, but whether or not you should use it depends on where on the spectrum between beginner and expert you think you are. It’s better to learn the hard way first, and to then make life easier for yourself with a framework.
Making sites reasonably responsive isn’t hard. It consists mainly of making sure the site adjusts itself for viewports of varying width. Pure HTML has done that since the start. It was only when designers started imposing printed-page-style design on websites that rigidity crept in. Still, the most strictly responsive sites even scale graphics down to size dynamically.
The only framework I’ve used is Codeigniter, but that’s a PHP framework, and all the HTML and CSS is left up to you.
Well, that’s my two cents.