Is just having a button with no writing too vague? So instead of "add new feed from url" it is just a little + in a square for example.

Yeah I don’t like that part. Need to shove it in a corner probably since I don’t think it’ll be used much

Could always have an «Edit» toggle on the top that exposes the buttons to edit/add/delete feeds and otherwise have it hidden to take less space

I feel like it should probably be easier to add new feeds. I think that fits into the philosophy of constant expansion within social media, you're always opening new connections and so you don't want to hide that behind another menu.