^^ These are a great questions.
Sims main weakens in ST/HDR is that he has a tough time getting attacking opponents to stop their momentum. Then if he is able to do that, he has to chase them down to knock down some of their life.
Higher level play with Sim can honestly take the just about the whole round to win. So you want to be really picky about how you decide to start your round. One knock down can lead to a very quick death if you are Sim.
You statement can really be broken down into three pieces:
What to do when a round starts?
What are good block strings?
How do I zone aggressively?
Be sure not to overlap the ideas of block stings and zoning. They are very different things. I think it is best to answer your questions backward b/c I feel that the way you asked part of your question shows you may the incorrect mind set when playing Sim.
What do you mean by ‘zoning aggressively?’
To be honest, I never see Sim as an aggressive zoner. I see him as a patient and controlled player. True as you get better with Sim, you can tighten your timing of specials an normals so seem at though you are playing quickly. But Fire Balls only travel so fast, characters only jump so fast, block stun lasts a specific amount of time. Sim only needs to be where he is supposed to be in the games own time. Not any sooner, not any later. Don’t force him (or his limbs) into any space where he doesn’t need to be. Just like pieces on a chess board, the fireballs and limbs only need to be, where they they need to be, when they need to be there :xeye:… LOL… You get my point. If you move to soon, you expose yourself in ways you may not have intended. Remember, Sim’s only invincible attack is his super. Any other time you choose to attack you expose yourself to the possibility of damage.
Playing to fast with Sim leads to a quick death. When I play against other Sims with other characters, I wait for them “to aggressively zone” me and then I am ALWAYS able to score a knock down. They leave to many holes in their zone I can always squeak in or gain a knock down.
Here is an example:
If I am Cammy I will gladly lose half my life to score a knock down. I follow it up, with an ambiguous C/U short > Cr.mp >Cr.mk> drill for a dizzy. Then jump in with Hp>HP>RH drill for a GGPO. But, I can can only do this if the Sim gets to predictable with his play…typically through aggressively trying to zoning me.
Have to get back to work…
I’ll get to the block string question with in the day.
If you can, I’d like to hear more about what you mean to aggressively zone. I may have misunderstood you and how you define the term. Somantics… you know how they can go… 
As always, I am certainly no expert. So I am not sure if this will help you. But, thanks for listening to my POV. :china: