Dont continually short hop bair, that'll get easy to read. You should be jumping a lot and mixing up how many times you jump each time youre in the air so it will be harder for your oppenent to guess when youre gonna hit the ground. There's many other approaches (falling nair, short-hop nair, spaced fair, etc.) that you can mix in aswell.
Shiek Matchup: Crouch a lot so they cant grab or dash attack (both should result in rest), fish for up-tilt-rests when possible. Edgeguarding as actually the easiest part of the matchup, whenever you hit her off, just grab ledge as fast as you can, and a lot of the times you can do a quick bair off ledge if you can read when theyre gonna jump/up-b (reading this with a bair almost always results in a kill because they will either be too far off w/o a jump to make it back, or they will HAVE to up onto the stage, which you can rest/fsmash). But if you cant read their jumps well, then just wait on ledge and a lot of times they will try to get their poof so it will hit you on ledge, and then they will go high. If they do this, you have to do a normal getup on stage (just push up on the control stick) and without moving just hit cstick in whichever direction theyre in.
Marth Matchup: Again, crouch a lot. Basically if he whiffs a grab while youre crouching you can just tap jump and rest. I guarantee if you hit one of those the marth will get scared and not go for as many grabs, eliminating a huge threat in the marth-jiggly matchup (Marth's grabs can lead into tippers if you dont DI perfectly). Dont use nair much in the matchup, except for nair-uptilt-rests. Backair a lot obviously, since thats your only move that can compete with marth's range. Fair out of shield can punish the majority of marth's attacks on your shield, but just poke him with the fair oos, committing too hard with fair will get you punished. Edguarding marth is kinda tricky. Here's the best thing to do imo. Knock them off then grab ledge as fast as you can. Almost as soon as you grab ledge (timing depends on how far you knocked them) hit down so you fall off ledge, then immediatly jump back fair (using ctick so that you remain facing the stage). This will catch him if he does an early double jump, and will tap him back far enough off stage so that you can regrab ledge and roll up for the stock. If marth goes low on his recovery, which most will, the backward fair will keep you hot of range for hit up-b hitbox. If he up-bs and misses you, you can usually regrab ledge before he grabs it, and he will fall to his death, OR if he up-bs onto the stage, its a free rest.
Stages depend on each matchup, and personal preference, generally stadium is puffs worst stage imo, and the rest are all situational.