Add the option to activate this AFAIK unique feature in Google Keep:

When you start typing a new task show matching completed tasks in the same list in a drop down, letting you untick the box to mark them as active instead of creating a new task.

There is NO better way to handle things like a shopping list where you will often come back to the same items repeatedly and may want to save things like link to order the product, sort by shop etc and therefore can save A LOT of time by just unticking your already existing Task, with these details, instead of recreating it every time! Same with certain recurring routine (maintenance, cleaning etc etc that may contain multiple subtasks).

For this reason alone I still use Keep in parallel with TickTick. This is killer feature!

currently completed tasks are gone somewhere I don't know where it goes!

I would LOVE that. In many areas I like to leave the completed tasks on to be able to visualize the progress, but having to drag and drop them to the bottom every time is kind of a pain, specially since the drag and drop doesn't work great for me.

this is a nice convenience — as an option