First of all, thank you both for your tips!
First thing I would work on a bit would be implementing some short hops and fast falling.
This is something I've been working on slowly, short hopping with Sheik is pretty difficult and usually when I'm mid game I can never really get it off, I can shffl perfectly in practice mode or against CPUs, but with pressure on me I can't ever actually pull it off, except against Peach, because I die if I don't, haha.
I like your use of aerial needles. It's also good to throw some ground needles occasionally; especially to force that Marth to approach.
Recently (as of the last 2-3 days) the Marth I play with, Dr. R, has started approaching with shffl'd nairs, and I can't find a say way to get through it. I'll be posting videos of this soon and hopefully you guys can help me out.
It's really not safe to continue dash attacking their shield and even ftilt can be punished if misspaced.
This is a problem I have on most of the cast. I can't really approach Marth with aerials or else I just get tilted/smashed back, so I'm not sure how to come at him on the ground, or even how to get in when he's coming at me. I end up resorting to dash attack because it's fast but I end up getting sheild grabbed/cc'd, etc.