You're probably spacing too far away if you're having trouble hitting Marth in between his hit boxes. You need to be just close enough that you can dash jc grab or running shine, but far enough to where you won't be threatened by his fsmash.
I've found that most low level marths will dash when it's time to grab, and usually stand still or jump when it's time to attack. Higher level Marths are a bit harder to deal with because their wd spacing is much better, as well as their patience, so you have to maneuver around and force him to try and read your movement, and punish him at dead angles when he commits to an option.
It's all about spacing in that mu
Another big thing is timing. You want to know exactly the end frames on all of his moves, because with marth, even though his attacks come out fast, they usually have a decent amount of lag between when he can throw out another hit box (most notably is fair, which if you space it close enough you can dd > Nair and pick him apart while he's in the air, similar to what leffen does vs hbox's bairs)
I recommend playing marth vs a good fox to kinda feel out how much of a commitment marth is actually putting in when choosing an option
Edit: wd oos is essential in this mu. Marth either respects it and so you can approach with overshot aerials, or Marth throws out a move to contest and you wd oos. If you think or if they're known to grab, you can do fade back nairs, drill in place, Nair in place, wd in jab to catch his dash in and force an sdi in to a follow up.