Bair actually has a comparable knockback to FSmash, but fsmash is charagable and hits at a slightly lower angle. The reason I always bair over fsmash, especially against opponents that actively try to mash out of stun, is that you can intentionally sour spot bair and on survival DI you can get a guaranteed fair/dive kick, which both happen to kill reliably as well. It is more of a precaution in a gray area where you are not sure your fsmash/bair will kill outright. Also because while Fsmash does have a huge range, stunning targets below the stage, or in awkward positions don't always lead to FSmash. Alternatively, I will Uair/Nair when I believe my opponent will not be killable by FSmash as sort of a 50/50 DI mixup. I am still not very accustomed to ZSS new throws and what is entirely possible outside of high % guaranteed KOs vs floaties with dthrow, so I would say most of these stun habits are bad on my part, and there are likely better followups at most %s out of uthrow/dthrow.
Dive Kick is my favorite ZSS move, and I believe it is highly underutilized. The move is stronger the closer you are to your opponent when the moves active frames start. In fact, if you start Dive Kick with your opponent inside of the starting hitbox, I believe it is stronger than sweet spot bair and fsmash.
There are a few people that have gotten decent at teching the initial dsmash hit on edgeguard if they are climbing a wall with firefox for instance, and it is also techable as soon as you leave stun.
My style is not optimal in the least, and most of what I do is experimentation from 2.5 when ZSS had an abysmal grab game and you had to be slightly unorthodox to get some kill opportunities.
