I would say Studio itself is reliable for general UI designing, as there’s a lot of libraries to ease UI development such as roact, rodux.
However, what you’ve said in the third paragraph is a bit false, the size of the UI does not affect the cleanness of the UI.
What does actually affect is the layout itself, even though you have small components but you stack them up together when there’s a few buttons can be combined into one, this is considered bad UX. Additionally, I want to let you know that large UI does not mean it’s clickbait. You use large UI component to show how important that content is, something like error screen should be the most important, same as promotions. And, there’s really no point making it draggable, if it doesn’t take up a lot of space or overlay with another core UI (you should’ve moved the UI to somewhere else then 
)
Apart from that, you might want to use “tooltips” for the button feature, only make it pop up when the user is hovering at the button.
Overall, it’s pretty nice and user-friendly. Keep up the work.