Touch everything. Everything. Touching is the best way of temping.
Fish is usually very definitive about it's done point. Make sure you use your eyes to check for a band of translucency on the side of a fillet or steak. You want the fish to not mush or feel hard when you touch temp. Like most proteins, you want it to spring back. You want the "flakes" to barely open when you press, not completely separate; if you've got definite flaking on both sides of the fish, it's overdone.
There are a couple of keys to breaking down proteins.
Most important, you need a VERY sharp knife. I cannot overemphasize the importance of keeping your meat knives SHARP. You want them at least sharp enough so that they won't force you to hold them too tightly, or use force to cut. Using a steel is not sharpening. If you don't know how to sharpen, let's talk about that.
If you're cutting from primals, whole fish, or whole birds -- they're going to "tell you" a lot about how they want to be cut and trimmed. Use your off hand to feel for the natural places where they want to be trimmed or broken.
Make sure you trim all silver-skin, gristle, and connective tissue from red meat. A little fat for flavor is nice, but something tasteless, too tough to chew, and difficulty for a diner to cut out herself -- not so much.
It helps to be very fast and confident with fish. That's something else which can't be over-emphasized. Slow, little, hesitant cuts make for ragged surfaces. No thank you.
Not all fish cut alike, knowing the particular species makes a lot of difference -- it comes with practice.
Always keep your knife straight in the cut, without letting it twist, wander, or cut on a bias.
Don't "saw" your blade back and forth when you're steaking, chopping or otherwise portioning. Keep your portion sizes very consistent. Consistent portion sizes will help on the grill too.
If you're doing most of the protein prep, your chef's knife isn't a good choice. Get the right knives. They don't have to be expensive. Forschners are not only plenty good enough. but "gold standard" for meat work. A blade which is a bit too long is better than a bit too short -- at least for me.
No on, off and back on again -- at least for the restaurant you describe. Yes to leaving enough time to rest proteins. A big fat NO to pre-cooking. It may be okay for catering outdoor steak dinners and other volume situations; but it is anathema for fine dining.
The semi-exception to no on and off: Anything too thick to cook quick relatively quickly should be marked on the grill and finished in the oven.
Touch, touch, touch. Sharp, sharp, sharp.
Hope this helps,
BDL