My only advice would be to go full NextCloud at first for simplicity instead of trying to integrate it with other services.
That being said, if later on you're looking for a way to store images I'd highly recommend Immich, I just finished setting up my own hosting setup a few days ago and it is gorgeous.
I always take out those paper ads that are attached to the top of the seat if front and turn them around so I'm not staring at them for a 5 hour flight.
Not for polybar but you might like to look into SwayNotificationCenter, I use it as the notification daemon of my hyprland setup and it has all the previous notifications in a menu with a do-not-disturb mode. I use a keybind to open up the menu but you could add an icon to polybar that when clicked runs swaync-client -t to toggle the menu.
My only advice would be to go full NextCloud at first for simplicity instead of trying to integrate it with other services.
That being said, if later on you're looking for a way to store images I'd highly recommend Immich, I just finished setting up my own hosting setup a few days ago and it is gorgeous.