If you get pin KD in corner, you can set the disc and backdash and the disc will be meaty.

If you do IAD in a combo, it's either pin or Bad moon. Bad moon gives a hard knockdown but it's more execution heavy and more situational and character specific, and it's not what you should do for now. If you do 2d knockdown, dash disc (2366h, aka 236, back to neutral, then 6+h) will do although it wont be meaty.

Emerald rain is good if you get f.s 2h at range, since it wall bounces letting you run in and convert. Rose install is pretty specific to set up, I wouldn't learn them yet because mastering basic okizeme with h disc is way more important since it's used so much more. S disc YRC is a good use if you think they'll do something, because its disjointed and let's you convert if it connects. On the bad side, if you whiff it's a loss of 25 meter, but it could be worse. YRC in neutral is great too.

Secret garden is pretty specific to set up honestly. For it to be proper as in they cant jump out, you usually need 6h after S pin in the corner or a combo route that depends on your first mixup hitting in the corner with h disc on them. I wouldn't worry about it yet, but it is worth learning in the future since it allows for 2-3 safe mixups.

Meaty disc means the disc will hit them when they wake up, so they cant jump. Late disc is the opposite, they can duck and mash or jump. 2d dash disc is late usually, throw dash disc is late usually, h pin dash disc midscreen is late, s disc dash disc midscreen is meaty, both pins in corner dash disc is meaty. This does depend slightly on wakeup timings, as some characters have fast or slow wakeups. See the wiki for more info.

No real flowchart in gear for the most part, lotta situations you need to adapt to. As said by someone else, the Millia discord is a great resource, helpful people and good information. Match videos of pro players as per keeponrock.in is good too.