So let's talk about dthrow tech chasing on fastfallers. I'm sure you already know some of this stuff but it might be helpful to write it all out for your reference and practice.
In the spacie match-ups, when you grab between 35 and 55%, uthrow's only guaranteed follow-ups don't lead into anything juicy, and unless they have really bad DI, you're unlikely to get more than a single aerial off of it and that's a huge bummer. Dthrowing and then reacting to both their initial DI and subsequent tech option will always get you something as long as your reaction time is good. At the very least you can always get a strong hit of dash attack, and while dash attack is pretty unreliable sometimes it has more combo potential than uthrow --> single aerial --> nothing.
The first thing you have to do off of dthrow is react to their initial DI. For our purposes there are three categories of DI that matter; full DI in (Fox lands right at Ness' feet, within his grab range), partial DI in/away/no DI (Fox lands outside of Ness' grab range, but if Ness did a full dash forward, he would go through Fox), and full DI away (If Ness did a full dash forward, Fox would still be in front of him). Luckily you only have to worry about two different inputs in response to both of these DI options.
If they DI full in at this percent, you can regrab them and get another free dthrow before they even get a chance to tech. Most Foxes aren't going to do this more than once, but whenever you see it, take the free damage and feel relieved that the reaction was so easy. If they do either partial DI away or full DI away, you have to input one initial dash (basically a foxtrot) towards their landing as soon as you are actionable after dthrow. This is to put Ness in position for the subsequent reactions.
If they did partial DI away and a full dash will make Ness go through Fox, cancel the full length of your initial dash by beginning to dash dance in place. You don't want to go through Fox in case they tech in place shine and you're not facing the right way to grab, so dash dancing in front of them leaves you open to react to a tech roll with a dash attack. Grabbing tech in place shine out of a dash dance is ****ing hard though, holy ****. Partial DI away tech in place shine might be the hardest to thing to react to. If they partial DI out and missed tech, continue dash dancing out of the range of get-up attack and wait for them to do something. If they get-up attack or neutral stand, dash attack them. If they roll in or away, follow their roll by ending your dash dance and continuing into a run in their direction and then dash attack.
Alternatively, upon seeing missed tech you can cancel your dash dance by shield stopping. You will then shield get-up attack and get a free grab (or shield grab neutral stand on reaction), or you can react to their tech roll with a wavedash out of shield into grab. This is a pretty hard reaction though, so I prefer the dash dance into dash attack method. If you don't want to risk things on a dash attack and you want a regrab, you can try it out.
If they full DI away, you will need the full length of that initial dash in order to cover their options because Fox lands so far away from you. If Fox does full DI away and tech away (which is super common), continue your initial dash into a run and dash attack him. If Fox does full DI away and techs in, dash back out of your initial dash and dash attack him. If Fox does full DI and techs in place, immediately dash grab him before he can shine or spotdodge. Full DI away to tech in place is one of the harder reactions here, and I'd say it's second or third most common so get used to reacting quickly.
For full DI away into missed tech, you don't need to start dash dancing. You just let your dash end and wait outside of the range of get-up attack. From here, if they get up attack, you can pretty much do anything to punish the endlag of it. If they're at the higher end of the tech chase range (55%ish), I like to fsmash. Otherwise, just wait it out and then dash attack or grab to continue the tech chase. If they missed tech and neutral stand, dash attack them. If they missed tech and roll away or in, dash attack them.
Alternatively you can jab reset DI away missed tech, but when your opponents start to get good they'll SDI up and roll out of the tech chase, which is a huge bummer. Doing rapid dtilt makes it harder to SDI out though, so you could try that. I know you're a fan of jab reset punishes but waiting out missed tech is technically more optimal in terms of coverage. You can also just skip all these reactions and dash attack as soon as you see missed tech. Use your best judgment here.
If you're a pivot master like me you can even punish techs in with dash back pivot fsmash on reaction. It's so cool. It's like the coolest thing ever. So Marth-y.
Here are some gfys that demonstrate some of these reactions in real matches, with slomo at the appropriate parts so you can see the reactions. In some of these cases Fox gets a shine out before I can dash attack him; I can assure you that dash attack will land before shine comes out in these situations, but my reactions are just a little slow. Sorry for not being amazing guys.
Full DI Away Tech In and Full DI Away Tech Away:
https://gfycat.com/AmusingWillingBordercollie
Full DI Away Missed Tech --> Wait
https://gfycat.com/BeautifulBronzeFruitfly
Full DI Away Missed Tech --> Dash Attack, Partial DI Away Tech In --> Pivot Fsmash:
https://gfycat.com/GorgeousInfamousCoral
Partial DI Away Missed Tech --> Shield Stop, Full DI Away Missed Tech --> Dash Dance into Dash Attack:
https://gfycat.com/EvenWillingBighorn
Full DI In --> Regrab, Full DI Away --> Grab Tech In Place, Full DI Away --> Immediate Dash Attack:
https://gfycat.com/HorribleFarflungChickadee
All of this stuff works on Falcon, but he takes longer to throw because of his weight so you have slightly less time to react to his initial DI. It also works on Falco, but his tech rolls are a bit longer so your reactions to tech away and tech in have to be faster. Practice these and pretty soon I won't be making fun of you as much for being bad at following up off of dthrow, and you'll get to do dank tech chase combos like CnB | Chandy the legend.
https://gfycat.com/MaleHealthyAegeancat
https://gfycat.com/PlainImpassionedIndigowingedparrot
https://gfycat.com/DisastrousParallelGalah