How can Ireuse sub-circuits in KiCad 5?

What you are looking for are sub-sheets (hierarchical sheets).

First, create a new one by going to "Place->Hierarchical Sheet". Once you click to place it, you will have the following dialog.

Here, the important thing is the "File name". This will be common for all of your copies of the buffer. The "Sheet name" will be unique for each copy.

Then, double-click to enter the sheet and place the common components similar to what I show here:

The important point in the internals is to use hierarchical labels (again from the "Place" menu). These will be the ports that connect the internal to the external wires. I've labeled "In" as an Input Pin and "Out" as an Output Pin. Don't worry about annotating right now.

Next, right click and leave your hierarchical sheet. Now, you have one copy of the sheet. You now need to import the hierarchical labels. Again from the "Place" menu, choose "Import Hierarchical Label" to get the hierarchical pins you created. Place the input pin on the left and the output pin on the right as shown:

Now you can make as many copies of the hierarchical sheet as you need, just keep the file name the same for each and give each one a new sheet name.

@pipe so maybe the question is in reality how to do it in KiCad instead of how to do in in version 5...

Of course. Up to now, I see there no great differences between Rev. 4 and 5. Maybe you got a bad nightly build.

For reusing subchematics at KiCad I wrote a tutorial, which can be found here:

https://www.mikrocontroller.net/wikifiles/a/ab/HierarchicalSchematicsAsBuildingblocksAtKiCad_RevC-EN_06May2015.pdf