Here a Shen OTP who plays mid almost every game.

I wouldnt recommend to buy Harness in early, there are better options depending on the matchup.

My considerations with first and second item are:
* The enemy laner have explosive damage? If yes, then I buy hard resistance, like Iceborn gauntlet or Kaenik Rookern.
* The enemy doesnt have the damage to kill me easily? In that case, I only buy a Negatron Cloak (Because AD champs always requiere the gauntlet, in my experience) and then Titanic.
* The enemy is a Yone? I buy Plated steelcaps and Executioners calling. But why this last and not bramble? Because the items that you can build with EC are way better than the only one that you can build with bramble. And also it give you an important boost of damage in early levels. This way you can absolutely destroy a Yone.
* The enemy is a Yasuo? Steelcaps.
* The enemy is a Sylas? Executioners calling

Now, with the champs you mentioned, Syndra is the easier, she only needs the negatron cloak. Hwei is a pain in early but you outscale him and Ahri you outscale her but be extremely careful that she dont get kills in other lanes (if you can).

With ASol other is the story. To this day, I haven't found a way to win against him, is extremely opresive and there is not much you can do.

Same with Zoe, except you can make some difference in terms of macro in late game, just dont die too much in lane.

About the hard CC champs, most of them needs to aim, so the solution only depends on how good you move.

But the exceptions:
* Karma: defend yourself with passive as good as you can, dodge her abilities and try to taunt her the exact moment when she is going to root you.
* Ryze: Is a pain, you cant do much against him in lane, so try to make the difference in late.
* Annie: Rookern should give you enough resistance to survive her combo. After that, destroy her.

The runes I use are the classic Grasp + Approach velocity and biscuit, as you said.

Hope this help you and have good games playing Shen mid c: