> mobile client sends a notification by default for any activity
This is a setting on a per-server basis, “Default Notification Settings” although it doesn’t effect “Highlight notifications” or poll notifications if the user has participated in a poll, so it leaves something to be desired.
> no active ruby bot library
I’ve used Discord.rb, and it may not meet the author’s definition of active, but it does exist.
> complicated subscription structure with unclear pricing (but probably <$20/m)
Discord is free. “Boosting” or paying at a server level is completely voluntary and unlocks mostly cosmetic features.
> mobile app is noticeably slower at everything
This one I don’t get. On my 5-year-old iPhone the Discord mobile app is pretty snappy. “slower” than what?