I do not understand this. You have Xubuntu 18.04? Am I right? As you can see at this link: https://packages.ubuntu.com/search?keywords=thunarx-python
Thunarx-python package od not available for Ubuntu 18.04 (bionic) at all. This is may be the reason why XFCE Insync thunar plugin (insync-thunar_3.0.23.40579_all.deb … available from https://www.insynchq.com/downloads) does not work.
So, you are using thunarx-python v0.3.0.1 from some PPA repo, but this version is not definitely compatible with Ubuntu 18.04. And this is the reason why context menu at XFCE still does not work.
From my point of view Insync developers does not really test the insync-thunar plugin on current LTS version of Ubuntu (+ all ubuntu based distros).
Any comments?