APFS folder encryption?
One possible way to do this without resorting to the command line is to create a resizable encrypted volume through Disk Utility. It acts as a folder that is decrypted during the mounting process. It supports APFS and AES 128/256.

To create a new blank encrypted folder:

Disk util > new image > new blank image > set Format: APFS > choose encryption scheme > set Image Format: read/write > OK. 

To encrypt an existing folder:

Disk util > new image > new image from folder > APFS > choose encryption scheme > set Image Format: read/write > OK.

This will create the encrypted folder as a .dmg that is decrypted when you mount it. Once mounted it acts just as a normal folder.