What's the simplest FOSS tool for me to remote controll my dad's PC for tech support? Both are running Ubuntu 18.10, and he has to set up his end himself.

ssh? :)

But a vnc server seems like it should not be too difficult to set up. Vino seems to be the default, and well integrated with the ubuntu desktop according to their wiki:
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Octodon is a nice general purpose instance. more