To save anyone else moving away from FTPserver a lot of time and debugging, WiFi Pro FTP Server by Medha has some issues.
1) It has no "Start server with android" option. Manually starting the server is always required. Apparently the developer never imagined like unattended scripts for backup.
2) While it can access the ExtSdCard, it cannot simultaneously access the ExtSdCard and internal storage. The root folder has to be manually changed. The real filesystem root is not an option.
3) There is no tasker plugin for any automation.
I just bought Servers Ultimate Pro by
Ice Cold Apps. I don't really need 63 different servers, just a well-implemented FTP server. But this looks good so far. It definitely handles items 1) and 3), and I'm still working on 2). It looks like it should, but I haven't found the right settings yet.