A lot of the time Marth isnt wanting to approach. Grabbing and tech chasing with regrabs is mainly what I do at low percentages because everything can get crouch canceled up until like 30 percent +. Down tilt is amazing tho
You can do dash dance > Down tilts to open them up to get the grabs which lead into your combos. If you run an extra step or more past your initial dash dance animation you can crouch out of it, making you able to do a down tilt. Try it by running a good distance until you see marth start moving his legs and then crouch, d tilt. Practice what lengths you can do this at. If you space it correctly tipper dtilts lead into grabs at low percents which can lead into lots of things
If you re under a platform when you get the grab, I usually Up throw, wait for their tech, react with either an up tilt or a jump back up air, and chain it together with more up airs or up tilts until they're too high up to do that anymore, then i jump and try for tipper fairs and nairs to send them off stage and etc. Can even read the tech from under the platform with a tipper fsmash through the platform on all stages but Dreamland if spaced correctly.
if you aren't, i usually just tech read them until about 35-40 with regrabs until I've either forced them under a platform so that i can up throw or until ive forced them near the ledge to try to throw them off and gimp.
You can also set up defensive walls with your down tilt that could result in landing a hit that leads into a good combo. Practice doing dtilt > short hop double fair / Dtilt >short hop fair, up air. Tipper fair has lots of hitstun for comboing.
For example, I'm controlling the space and I'm trying to push a Sheik into the corner with dash dancing and grabs in neutral. I'm doing this by baiting out her attacks, getting grabs and throwing her in directions that will lead her to the edge where i can either control her offstage or platform tech read combo her. I can throw in the dash dance > Down tilts in there and do short hop double tipper fairs directly after them with the correct timing and rhythm. You can bait out shield grabs with these because if you space the down tilt correctly they won't be able to shield grab you, and that's when that short hop tipper fair will land. Another way to punished that baited shield grab is to dtilt, dash away and dash back in and if they shield grabbed you should be able to get a free grab. If one of those tipper fairs hits and isn't shielded tho, the other one can hit too with proper spacing, and you've possibly got a combo started. I find that If you do land that first short hop tipper fair and its at low/mid percents, Its better to do an up air in place of the second fair (but only lower percents against floaties) , because it pops your opponent up for more tipper fairs/ up airs into a potential ken combo, Tipper combo, edge guard situation, platform tech read situation, whatever.
Dtilt is kool use it