PhantomJS and SlimerJS are two of my favorite tools for this kind of job, which fits well with your requirements.
My experience with building web scrapers ranges from static web pages for simple periodical report creation of product inventories and as far as dynamically loaded pages via JavaScript with bot prevention systems provided by Cloudflare and Distil Networks protecting: ticketing solutions websites, stock market, and Forex analysis. So the login page will surely not pose any problem.
I'm looking forward to further discuss your needs and answer any questions you may have.
Best,
Andy