It seems you did some research already but to make sure we're on the same page.
D-throw follow-ups can be a bit awkward since it's very character dependent, especially at higher percentages. Rule of thumb:
Low percents: a simple dthrow - jab will work wonders and is pretty consistent. Some characters may fall out of the wind variation more easily while some like Sheik and Ryu will be eaten alive by it, even a bit beyond low percent.
low-mid percents: dthrow-utilt or dthrow-frame/airdodge trap. Some characters you may have to start with dthrow-utilt right away instead of dthrow-jab especially if you have rage, if I'm correct. Someone correct me if I'm wrong.
high percent: dthrow-frame/airdoge trap or CHECKMATE (dthrow-levin uair). The checkmate, while reliable on most if not all characters, varies drastically on what percent you can pull it off based on the character. But it is a true combo.
As for Arcfire, just start with a simple forward short hop levin fair or levin uair, assuming you did it on the ground. You may need to run or not depending on the distance. Arcfire provides plenty of stun time so you don't really need to buffer the inputs.
Same things can apply to a close-mid range arcthunder and thanks to the new patch buffs, it has plenty of new follow-ups including dthrow (see above). But if you're struggling with combos, just keep it simple with levin fair, levin uair, dthrow, and if it's point blank, smash attack also work.
A lot of these combos aren't that hard, and as discussed earlier Robin isn't that big of a combo machine when compared to let's say Sheik. If anything the hardest thing may be checkmate, but again that has more to do with how it varies drastically of when you can do it on each character than actual execution. So just practice a bit in training mode (although training mode may read arcfire combos a bit strangely) and you will be fine.
Edit: NOOO! Dath!