He was definitely using SDI. I don't think I could have Up B'ed in time. It looks like I tried to act as soon as the animation was over
It seemed like you tried to act asap. I just wasn't sure about possible IASA frames on bair that could have been used. Bair to UpB will only work if you get a later hit of bair. Sometimes I achieve this by bairing towards them and landing the later hitboxes instead of timing the bair to come out on them. It is more forecasted, but as long as they are in hit stun it doesn't really matter.
If people can learn to consistently SDI bair like that then it may be troublesome in the long run.
Another option for low % follow ups could be fthrow or bthrow. If they DI the wrong way you get similar things as no DI on upthrow. If they DI the correct way they go pretty far though, this is useful at the edge because it sort of mimmicks Falcon's drop zone out of downthrow when he's facing the edge. We can drop with fair for sure, nair could probably work if timed correctly and either can send them pretty far off stage leaving us with good position even if they still get to recover.
I'm also not dealing with the same caliber of players as you so I'm not dealing with people who SDI my bair or uair. It does happen sometimes and sadly I just don't think there is an option that covers strong DI from upthrow into uair in some instances.
Against Toon Link it is worth noting that Downthrow follow-ups are very plentiful because his tech is garbage. It's so short that you can react to which way he goes and get more grabs/whatever. The issue with the bad techs is that since they go less distance DACUS will go past them and isn't feasible unless you somehow compensate for the distance.