Yup, adjusting proxy settings and disabling JavaScript are not possible with a WebExtension which is why those features had to be removed. There is the possibility that I may be able to add some basic cache features back in a future release, but they will be probably be somewhat limited given the lack of APIs I now have available.

Controlling the user agent was actually part of my other extension User Agent Switcher which I do hope to update as a WebExtension. However, this requires rewriting it entirely from scratch so I am not sure when I will have the time to do that honestly.