notification center interaction is pointlessly more difficult

Not sure how you are having troubles with it maybe due to being new to it, For me its the opposite where the android version works superb whereas the IOS is horrible from a design and usage standpoint

the volume level resets every time you plug into the headphone jack

Do you mean how it's separate to the having the Phone unplugged? for me this experience is far better on Android as it means I can have the volume set for when I'm connected via Bluetooth or when I'm using headphones.

google assistant won't open Spotify, and has a hard time sending texts and calling people.

It's a bit of a pain and kind of hidden, but you can set Spotify as the default music app and it works well. To do this just go to apps in settings and disable play music, then use the assisstant to play (song name) and it will give you a list of music players installed to choose as default. As for sending texts and calling people I have never had this issue.

twitter feed is layed out really annoying

There are lots of Twitter apps you can use to fix this e.g. Falcon

Once you get used to Android you realise the great advantages, after 4 years with androids I'm still learning things. I have had an iPhone 6 in this time and I just can't stand so many things on Apple that its impossible for me to go back since switching after the 3gs. As for responsiveness with the keyboard. Try a different keyboard app I'm using gboard the default Samsung one can act a bit strange.