Hi @Mark
Welcome to Wappler Community.
I too come from a similar background where I coded everything using custom HTML/CSS/JS along with 3rd party tools for some of the more advanced controls/options.
You can hand-code websites even with Wappler, but to use the App Connect framework & its various drag-drop elements etc, you absolutely need Bootstrap.
Bootstrap not only gives a nice UI, it takes care of all responsive stuff.. giving a good UX too.
This is was one of the biggest challenges when switching to Wappler. But once I got the hang of Bootstrap classes, everything became much-much faster and simpler.
There are options to change breakpoint widths in settings, but haven't used them. Following standard breakpoints has worked well for me so far.
Design panel allows for custom CSS units, so should not be a concern.
Never been an issue with Wappler.
You can read more about Others vs Wappler on other posts: