I don't find predestination scary. I like the idea of having been "decide(d) upon beforehand". I like the idea of someone ( God) planning for me, creating me , electing me and living with me because He wanted to (His will). Some people argue it (predestination) leaves no room for choice but if He chose me than He wouldn't be dictatorial and leave me no choice. There's plenty of choosing left to do on our part..Choose to believe, follow, live for... or not. Predestination just means He had a plan and I am/we are part of it!
I don't know why Paul would bring it up except as a comfort to the Gentiles b/c the Israelites/Jews were always considered the "chosen ones" and I think Paul is trying to break down any wall of exclusion ...not leaving any room for the Gentiles to say we don't belong or the reverse for the Isrealites to say you don't belong. Kind of like putting us all on the same side...
Perhaps Paul praised God for it in front of the Ephesians so they would also extend the blessing(s) to other Gentiles not of Ephesian background. Also because He was a Jew and wanted them to know He accepted them and was glad God included them in the blessings through Christ Jesus.