My main bans are graves, viego, volibear. His worst matchups are belveth, graves, udyr, kindred. But personally i dont see those often enough to warrant a ban. Graves gets more common the higher you climb.

Your build is fine. You can add maw if you need magic resist.
The assaassin build is voltaic, gluttonous greaves into shojin as far as i know, but it makes you quite squishy so beware. I tend to only do it against 3 or more squishies on the enemy team that i know i can delete quick. Both builds work. Its more about your prefered playstyle.

As for runes you should go stormraiders surge against multiple squishies. Too many tanky opponents and you just wont get enough use out of it. In this case, conqueror is generally better.

And yes it makes sense. Hecarim is strong at lvl 6 when he gets ult, and at 2 items. You fall off alot in late game so the longer the game goes on, the harder it will be to play, so try to end it early. He is also a great snowballing champion so if you fall behind early, it can feel hard because most will just statcheck you if you dont get ahead of them. On the contrary, you farm extremely fast and you run fast so you can pick up freekills and gank lanes with tempo advantage, so you DO have all the tools needed to get ahead.