As someone who has been at the same place, my two cents.
When I developed my first app, my knowledge of css and javascript as very basic. I could identify working code and worked by cut/paste.
I was overwhelmed by the number of options available compared to backend dev, which is pretty much stable. I posted on HN and the consensus of the majority was to use bootstrap as it was widely used.
For a beginner learning on your own, you need to have lots of support when you run into walls. Because of the large community, Bootstrap has a wide userbase and most usecases are covered by some tutorial or a question on stackoverflow. JQuery is well seasoned library, so less bugs and more support.
Also, your need is to release your SAAS app as soon as possible. A framework or library that does not throw you any surprises and has a good coverage of components is your best bet.
My javascript interactions were not too detailed, so I stuck with JQuery. Your mileage may vary.
Now I have had some validation for my product, and I realized that legacy browsers form a very small part of the visitors but a reasonable portion of them were on mobile, I have set down the path of making the app mobile friendly.
Armed with the knowledge gained from bootstrap and jquery, I have a general plan of what layout and interactions I need, and also how to go about achieving them. Currently I am reimplementing them in sass with a few libraries and vanilla js. The page size has come down from 312Kb to 35Kb. But if I had attempted pure js/css at the start I believe, the project would have been mired in unnecessary complexities.