If he's consistently hitting you with a move from the full stage away that means you're consistently dash-dancing the full stage away. You can't just sit in the corner and wait for him to come to you all the time. Dash-dancing works because you threaten space without committing very much. If you never move towards you opponent, you're not threatening them; you're staying in the same place doing the same thing, so they can simply attack you knowing that you're not going to do anything about it (and if you're literally at the edge of the stage you have nowhere to dash back even further to avoid an attack). Not only are you being predictable, but you're showing that you're never going to try and hit your opponent. Do you see the problem? Why wouldn't they just hit you in that situation? What would make them throw out an attack that doesn't quite reach you that you can punish when they know where you're going to be all the time?
Just watch good players to get an idea. Even players known for sitting in the corner and dash-dance camping, like Mew2King, will very often move into their opponents space to bait them into attacking,
then dash
further back to avoid the attack and subsequently punish them. And, if their opponent doesn't try to approach them, they can then move forward and try to take their space or attack them. That was basically my whole neutral game plan forever: dash-dance to bait them into approaching, if they don't approach then approach them to punish that and thereby make them approach more, if they do approach then move backwards and punish. It's simple, and obviously it won't be sustainable to just do it
that flowchart, but you see the fundamental idea. It's a good way to think about things to start off.
Also, from that distance you can crouch-cancel on reaction. You have more time to do that than you do to shield since you have the duration of hitlag to react in addition to before the move hits you (which is all you have if you shield). You see top players crouch-cancel or DI pretty much everything unless they get caught in the middle of another input when they're not expecting it (like, they just started to dash back and their opponent throws out a forward smash so they DI full away and die early), because they're always primed to react to things so that when they do happen they'll be able to react faster. Often they won't be preemptively DIing (although that can be the case too), in neutral at least. And something like max-range dash attack is amongst the easier things to react to if you're ready for it. It's a good place to start learning. But dealing with dash-attack as such isn't really the problem you have (though you could also get better at dealing with that specifically), it's simply a way that your more fundamental problems are being exposed. On the same note, this isn't a Marth issue; other characters have their own way of punishing what you're doing.
right now i end up submitting to his playstyle and dash attacking him back which feels grimy and unprofessional
What does that even mean?
Would shielding work (wont he just run up and grab if I shield it? Also wont it be hard to react to?)
Well yeah he could run up and grab but not from fullstage. Although you wouldn't want to shield from fullstage because there's no reason to do it that early, so it's kind of a moot point. From closer range where you can't just pure react to everything it becomes a mixup. Sure he
can just grab to beat you shielding, but he can also throw out a move to beat you not shielding, you know? The point is that he can't just react to beat whatever you do, so you have to predict what he wants (based on the risk/reward of each option and his habtis) and choose the option you think is appropriate.
You can CC the dash attack and dtilt if he tries grabbing. You can also try fsmashing and stuff his approach. Dashdancing isn't as good as a lot of people make it out to be unless you know the purpose of it, so if you try just dashdancing but he's hiting you anyway, you should try other things and only dashdance when you can't think of any mixups, especially as and against newer players.
CC dash attack is good. But getting hit out of dash-dancing doesn't mean dash-dancing is a bad option, it means you're doing it wrong (in other words, dash-dancing is highly variable with lots of nuance, and not a singular, static option). Certainly in this case it does. You should learn to dash-dance better, not choose another option. Sitting in the corner moving back and forth never actually doing anything is only dash-dancing in the most literal, superficial sense.