Marth's move set pretty much out-ranges everything Falcon does.
Accept this fact, and stop trying to beat him in neutral with Nair, you'll die.
That doesnt mean never Nair, It just means choose when you Nair sparingly, mostly off grab combos.
In neutral, you want to dash dance at a safe range, and punish his whiffs.
For example: Many Marths are rowdy and over aggressive because they know they out range most characters.A lot of Marth players will do a fast-fall approaching Nair, you dash dance outside the range until after the aerial is done and run In grab> combo punish.
Run up shield Is also great In this matchup.
Run up shield>Shield grab Marth's poorly approached aerial
Run up shield>Up-Air Out of Shield
As for combos grabs are bread and butter, Down-throw regrab Down-throw Nair Up air at 0 sets up for K O
At about 70-80 percent you can Up throw Knee or Down throw If preferred
Most marths love to spam F-smash, you need to learn how to predict this to avoid them, a good sign an F-smash Is headed your way is when marth has his back towards you, and is slowly walking away, huge F-smash red flag.
Marth's down tilt Is an extremely fast move, you should try to read his dtilt patterns, you can dash dance>approach stomp above marth, slightly drifting past/behind him, this way you'll get a stomp, If he shields, you can dash back grab punish.
You want to avoid being above marth on Platforms , and the same for him Is true, Both marth and Falcon cover above them very well.
When you catch a Marth on a platform above you, do Instant Up-airs, many In succession, this will force marth to shield, then you can either do a knee and It can shield poke him , or continue with Up airs for guaranteed pressure
Dont be afraid to edge-guard marth, You have many options.
when Marth Is recovering he cant do much to be defensive, especially without a double jump. If he Is this close to the stage>
https://i.imgur.com/O5r9nGm.png
you can easily jump down>fastfall>back-air> recover.
If marth Is a bit too low or far for that, you can go the safer route with the infamous marth killer, which Is a light shield at the edge>marth up B's into you>Falcon Grabs ledge> Knee.
OR
Refresh ledge Invincibility>marth lands onstage>Knee/Stomp>knee depending on percent
It sounds Like you need to brush up on your marth combos, I suggest experimenting In 20xx or offline, and practice when certain combos work.
Understand that this matchup Is mostly even with a slight edge to Marth solely by range.But Falcon absolutely 0-deaths marth consisntently In the right hands, you can do It, stick with it, study, watch Wizzrobe VS marths, and Good luck.
Regards
-Timothy Price