I started off like you on the backend development. When I had to build small webapps, I looked at the various frameworks and then settled on KnockoutJS. KnockoutJS is a very small and simple JS library. This is several years ago and at that time I didn't even know Javascript. The reason I say that is because even for a JS newbie, the KnockoutJS library was simple to use. (I read somewhere that MS Azure used KnockoutJS for their frontend dashboards in their earlier version. Not sure if they still do.)
If you are comfortable with plain HTML + CSS + Javascript, adding KnockoutJS will give you a minimal app building capability that you will find useful for several usecases.
You get to manage your application's data model in javascript on the client side and the UI can be bound to your datamodel (bidirectionally) without having to manage it yourself. That is more than sufficient for many usecases.
If you don't want to be fiddling around with CSS too much, you can use something like Bootstrap or Material Design's CSS templates to get some decent looking UI (without getting lost in CSS minutiae).
For your development workflow, start without any bundler (like WebPack or Parcel) first. Once, you have your application working, you can then introduce it as a final step.