You have to punish those fairs more and make it feel unsafe for marth to go in the air.
Blizzard is good for this a lot of times, which you try to do, but then you get hit by the fair because you're in front of nana.
Something wobbles does against marth is dash dance desync blizzard + shield stop with popo.
So nana blizzards and popo has his shield up.
This is nice against characters with big moves because then popo doesn't get poked through the blizzard. Especially marths because they like to fair or sometimes dtilt you through it if you space poorly.
I think you should stop roll desyncing so much as well, because every time you go for it, he catches you. Roll desyncs are ok in a few positions, but usually they are a pretty bad habit. They are slow, which makes them risky against fast characters like marth or fox and they give up a lot of stage, which is important against marth.
His fair approaches are very predictable, so what you can do is wavedash into him with your shield up and grab after the fair. Marth's aerials are very unsafe on shield, especially if they are not spaced well.
When you do this you have to look for sh double fair or doublejump away as mixups. For the double fair just keep holding shield, it's less safe than a single AC fair. For the double jump, just wavedash->sh uair or usmash depending on percent.
Marth can also double jump and airdodge after 1 frame, which lets him waveland out of sh fair. It's pretty good, and can be hard to react to, but no marths do that yet for some reason. When he does this there are a couple ways you can catch him, but it can be a really tight window if he does fair->waveland back->dtilt, so I try to force him into the corner of the stage.
On that note, I think you should try to trap marth more. he kind of just jumps all around you. If you are aware of his options and use uairs and shield well, marth struggles a lot in the corner against ICs, since he needs space to deal with them. If he goes to the ledge for invincibility, I just stand back a bit with projectiles and wd uair to catch if he tries to go high to a platform.
