This amount of separate software projects from different people to learn, to track and maintain just for a single website? Has the world gone mad? I remember the times when we didn't had to duct type 20 buzzword of the month libraries to build something useful, :(
These are the only pieces you need (besides CSS, HTML and JS) to get a large frontend heavy web application off the ground:
1. A web framework. OP used Flask, which is a microframework. Not a good choice for a large project. To gain feature parity with full-featured frameworks, you end up having to use tens of third-party extensions of questionable quality. As I mentioned in another comment, Pyramid would've been a better choice.
2. A frontend framework. I haven't personally tried all of them, but Ember.js looks like a good choice to me.
3. A database. Forget about fancypants NoSQL databases. Just use MySQL, PostgreSQL, MSSQL or whatever RDBMS you find convenient. Relational databases were built for a reason, and people recommending MongoDB or whatever are doing it because either (a) they want to be hip; or (b) they have a vested interested in the success of the NoSQL movement; or (c) they slept through their CS classes and have no idea that NoSQL databases are rehashed versions of thirty year old technologies which were thrown out the window because they were so terrible at maintaining data integrity.
That's pretty much all you need. SASS, RequireJS, CoffeeScript, Handlebars etc. are completely optional and just there to make your life easier.
To be honest, the only frontend-heavy apps I know of that are complex enough to warrant the use of seven billion hipster libraries are Google's apps. If you're not writing GMail or Google Docs, don't get sucked into library-itis. The time you waste learning ten libraries (five of which won't be around two years from now) is better spent improving your app. You'll end up with 200 extra lines of code, but at least your codebase won't be 98% third-party code.
It definitely can be overwhelming the number of libraries and choices out there. It doesn't mean you have to use all of them, though, each component serves a function that you may or may not need.
One thing is for sure, though, the bar has been raised.
