You can find broken internal links from within the Search Console. You need to attend to each warning appropriately telling Google that you have fixed it. Having excessive 404s will hurt your site if they are really 404s, because anyone could escalate the 404s by pointing randomly to pages that don't exist from external places, which is why this is not that big of a deal - but should be looked at.
1. AdWords Keyword Planner - It's still the standard, although Google keeps making changes that just aren't helpful. I get that they want us to treat closely-related keywords in such a way that we're not creating multiple pages when we should just have one, but I'd appreciate it if they'd still break down the volume for each keyword that makes up a group (or at least list the keywords they're clumping together into a group).
External duplicate content is when you copy-paste content from other websites. Usually without placing a link to the original source or using canonical attribute. Google dislikes such content for one simple reason. It doesn’t add any value to end users and the Internet in general. If you have to choose between copying articles from other sources or not having any at all, I’d suggest you choose the latter.
SEMrush is a very useful tool for both researching competitors when starting a site or for growing an established site. I really like to find weaker niche sites that still seem to be ranking for lots of keywords; SEMrush helps me see what they are ranking for and what I can potentially target. You can also see what keywords you’re on the cusp of ranking for with your established site - another very useful feature.
Yes solid keyword data dramatically changes the game as far as rankings go. I have seen many tools that provide Cost per click stats and meaningless charts that confuse most users. the data Jaaxy provides helps drive “organic” traffic to your site on a consistent basis which will out perform PPC sites every time. I like how you say thinking like the end user, as I feel that way when I search. I dive into what people are looking for and it really helps drive content idea’s. I have seen great results from that.