The problem is that you need to activate both the .esp file that comes along with the mod, and the .bsa files, so that the data in these archives can actually be read☨.
While the .bsa's can be activated through Skyrim.ini, activating the .esp there won't work (the game won't even start if you put anything else than .bsa files in the
As you found out already, to activate .esp files manually, you have to edit or create the plugins.txt in a different folder. This file is located on the drive your OS is installed on (usually C:) (I include Windows to somewhat extend the range of users who find themselves in this rare situation).
These are the default paths:
resources/transgaming/c_drive/Program Files/The Elder Scrolls V Skyrim/%USERPROFILE%/Local Settings/Application Data/Skyrim
C:\Users\%USERPROFILE%\AppData\Local\Skyrim Special Edition
- In this folder, find or create the file plugins.txt (make sure it's actually a .txt file with the name 'plugins'; make file extensions visible to be sure).
- Open plugins.txt, and add the filename of the .esp file (i.c. SofiaFollower.esp).
- Navigate to the plugins.txt file properties, and set it to 'Read Only' (or running Skyrim could change the contents).
- Be sure you have the .bsa files listed in Skyrim.ini.
You will have to start a new game for mods to get activated. I tested it out, and Sofia is working for me.
Please let me know if you followed all these steps and you still have no luck - I'm sure we can figure out what goes wrong.
☨: Or, rather, the .esp file injects the actual changes into the game, and takes additional data from its corresponding .bsa files. This is why texture replacers can consist of only .bsa files (they are read and loaded after the vanilla files, and whenever they have the same name, the file - like a texture file - that was loaded the last will have precedence), or mods that simply change gameplay might come in a single .esp file (since no additional resources are necessary).