It's not usually complicated at all. You just download an SFTP or FTP server app from the Android store, run it, tell it the username/password and port, then connect to it from Opus (or another SFTP / FTP client) using the same details.
Doing it any other way will probably end up being more complicated.
- Bluetooth support on Windows is a complete mess (although better than it used to be, and maybe OK if only sending a small number of files).
- MTP is great if it works with your phone and OS combination, but that seems to vary greatly between phone vendors and OS versions, and it often never works that great as there are problems in the MTP stack on the Windows side as well.
- FTP / SFTP are old, well understood protocols, where one piece of software handles everything at one end and another at the other end, without having to worry about relying on broken OS features. They generally "just work", at least over a LAN. They only become complicated when trying to put a server on the internet and having to worry about NAT, firewalls, etc.
That's a question for the mobile phone industry. I have many others for them. 