Some other stuff:
While laser grab is good, it's not really guaranteed, and if you're jumping close enough to laser grab, Marth can hit you before the laser is out, so try mixing in some dash dance after lasers.
When you get Marth in the air and pillaring doesn't seem practical at the time, just keep juggling him with nair uair or fair depending on which has the most appropriate hitbox, and either try to combo for percent, or get him into an edgeguard position. If Marth's coming down from the air, and on net, you're moving towards eachother horizontally, you can shield and cover anything he's coming down with, unless he's already charging neutral B. You can also often just keep knocking him back up, and if he's trying to time things against you going up in the air, then if you were say fullhopping, you could short hop and fastfall back down and uptilt or double jump to mix up his timing.
In the neutral, depending on percent, Marth's going to be very dependent on down tilt and grab. He can only really safely use fsmash as an anti-air, jab is potentially punishable on hit depending on positions, and side-b suffers a similar problem at lower percents. To cover aerial approaches over d-tilt, he's going to have to use autocancel nair, or really low fair. If he's approaching with either, you can shield and punish, or usually dash dance punish. If he does them retreating, he's giving up space, and you can likely either ftilt or fsmash him. Between auto-cancel bair and later bairs, you cover most of Marth's approach options, but be wary of run up shield. While lasering a lot is good, Marth can dash under and potentially grab or dash attack. SH dair generally stuffs both. Laser -> aerial should be used sparingly against better Marths, as they can aerial out of shield and not have to give up stage control, wavedash forward at certain ranges and go under anything but dair (and you shouldn't be doing super early aerials unless it's part of a punish), or wavedash back and punish, and that's all if he chooses to shield: he doesn't have to, and can gain significant ground safely if he CCs lasers.
When pressuring a lost of character's shields, your short hop moves your hurtbox out of the way or a shield grab. This isn't the case with Marth. If you cross him up on shield, you're usually in the clear as his out of shield options from behind are not the fastest and very punishable. Be ready to shield, and watchout for wavedash fsmashes. While I wouldn't really recommend this, you can use jabs to force royzones in his grabs from very close up, but he can counter this with appropriate shield DI.
Most Marths love dash dance grab, and just walking forward with a move, even a grab of your own can be hard for them to work against if they don't have consistent pivots or if you can adjust speed to properly deal with them needing to fox trot or not. Jab can also work wonders, but it's still not the safest move on hit at lower percents. Generally, retreating quick aerials and shine grab are very good, especially when you're closer to center stage. Also, be aware that lower limbs can not be grabbed in this game. After you've hit Marth's shield with an aerial and a shine, a sh nair at max height can poke, and if he angles his shield to prevent this, he can be dtilted. Hit it a few more times, and you can go for aerial doubleshine usmash, which isn't as hard as it sounds, and can break shields. Spaced ftilts are also really good.
If the Marth spaces grabs and other moves with shield stopping, spotdodge shine isn't going to work as well, but you could try to dash dance away, grab him first, jab, or downsmash. Downsmash can also counter downtilt without going into the air since your legs are invincible until the hitbox is out, but it doesn't really challenge too many options. Rolling can also be done in situations where you don't think you could spotdodge towards (please don't roll at Marth unless he's doing something fairly laggy, or it's akward for him to properly cover either because of moderate lag and royzone postitioning, or he's moving towards you and committing to it).
When Marth has to recover low, Marth killer will usually allow you to cover most options, and if he lands on stage, assuming he didn't ledge cancel or get assisted by a stage hazard (wispy's wind), you can bair him back off and try again, or dair him into another aerial if he's DIing for the bair. If Marth is close enough horizontally to the stage that his sword would pretty much have to hit through it, you can jump backwards over the up-b, before or when it's coming out, and grab ledge before Marth can do anything. If he's recovering high, treat it kind of like any situation where he's in the air, don't be afraid to wait, and if you know you can, hit him back.
When recovering, higher phantasms aren't properly covered on the ground generally if he tries to intercept, and if you can mix between that and shortened ones to then you have much better odds of getting back. With that said, phantasm does have a lot of endlag, so if he doesn't get hit or try to hit it, he can punish fairly easily.
While uthrow is usually the best throw, bthrow and fthrow have followups of some sort if you respond appropriately. If it doesn't setup for an advantageous situation to approach with an aerial (ideally nair) you can do a tilt that isn't dtilt, or a smash that isn't dsmash usually, just go for what visually looks like it should connect at the time. If you aren't confident in any of those, you can simply laser, or go for a regrab.
For covering techs, dsmash to cover missed tech or tech in place is good, fsmash to cover techroll through is also great. Techroll away, unless you started very close, tends to be harder to cover on reaction with things like this. You could pivot fsmash or wavedash after it, but you can also just go for a nair at higher percents. At lower percents, to extend a combo you can replace instances of dsmash with utilt, shine, or dtilt, and fsmash with dair. You can always land the smashes on platforms though, can space to hit dsmash for all options.
A bit of advice I have for all matchups in smash. Try not to do things that make it hard to cover options in neutral that would be easy to cover if you simply did nothing.