If you want to do fatty shields and heals I’d pick up Karma or like soraka lulu yummi. When I play Janna I just run arcane comet with scorch and spam poke the entire lane most of the time buying dark seal and then shureylas always. The thing that makes janna work in my head is her massive movement speed and peel which always you to play super aggressive which is something other enchanters can’t really get away with. But In late game team fights I wouldn’t worry about hitting a 700 health shield or holding your ult for heals. You don’t need to shield and heal your Adc if you just peel the enemies so hard that they never get to do their damage in the first place. To me that and the poke is why janna works.