Some of this is probably obvious as a mario main but imma say it anyway
Dthrow fsmash only works if they do no DI/slight DI. Mario can cover forward and behind DI.
But it's still a solid option kinda like upthrow>rest. Works best on spacies at 100%+ and sheik at 0-60ish%.
Fsmash is significantly harder to combo into as doc. Only one I can get consistenty is reverse upair (move into them backwards and hit with the end of the move). There will be opportunities for fsmashes depending on DI, mostly from upair and especially when comboing with platforms. I've gotten dair into fsmash a few times also. Most of my fsmashes come from missed techs or tech chases (occasionally roll read too i guess).
Dsmash is easier to combo into because it comes out faster, so any situation where you need extra slide, or need to be a bit faster, then dsmash is a great finisher. You can do dthrow>dsmash against spacies as a mixup at lower percents, great when there are platforms around or you want they offstage for a quick gimp. Jab > dsmash is a staple jab mixup and can be brutal at high percents or near the ledge.
Upairs to fair is great but a pretty tight window. Something you go for when you think the time is right lol, there are too many variables to give you a concrete percent. You can also do upair to reverse bair which is pretty cool, or if upairing backwards just reverse upairs to bair.
Doc is best at the ledge imo. Bair and cape are amazing against high-tiers. Biggest thing to remember with doc v mario is doc's cape reaches lower. This allows his to cape most sweetspots from onstage. Bair is great against spacies and falcon, and can help kill them at ridiculously low percents. It's also great as a surprise against any other char. Pills are great for limiting recovery options, and pretty funny against spacies upb. I generally throw 2 fullhop pills for anyone recovering low, can kill them with just that and an instant ledgehog (mostly marth). Doc also has ftilt (all angles), uptilt(early hit), nair (use like sheiks nair and create a wall, or use onstage), when you don't have the time or positioning to cape/bair.
Doc's upb is pretty notorious for stage spiking. If someone tries to go below the stage to hit you when you are close enough to upb, it'll be like a reflex to hit them. A good opponent could bait this though and leave you vulnerable on stage.
Upsmash to fair is a thing also, I don't know the percents for this, but just play around with upsmash (upsmash oos is great for hitting behind) and you will see the opportunities. Uptilt (late hit) can also work the same too, and dash attack (late hit).
Nair can be a finisher also if hit late enough. As mentioned above you can create a wall with it, I use this against spacies sometimes when I expect them to side-b. You can use it when you think someone is going to airdodge (like a ledgehop nair onto the stage), you can even finish a combo with it. You can land a stagespike with it sometimes with a quick ledgehop nair against a recovery, this is usually for style points honestly, but can sometimes be your only option when they try to recover to the stage.
I use Mario for falcon, but I'm more comfortable with doc against everyone else. Reason I use him for falcon is because fireballs are much better at obstructing his dash dance, uptilt is easier to combo from which is important as its an amazing anti-air, and fsmash has nice range and helps in many situations. The increased speed in aerial landing and jabs is also nice. I find it's best to play really aggro in that mu, and stay close to him as much as possible, but you have cc and uptilt when there is distance between you.
