Does a trade union of YouTubers exist? Would YouTubers see a sense in having a trade union?

For example, to establish policies how much the value of their contribution is.

The controversy here is, professional YouTubers make a bet for their income on the platform and have to accept its conditions, on the other hand YT has enough dominance and yet dependant on YouTubers making the bet their content will get them enough ad turnover.
Yes. In 2018, the German YouTuber Jörg Sprave established YouTubers Union (see also FairTube).

In 2019, it entered into a partnership with IG Metall: IG Metall press release, news stories by Observer, Bloomberg, CNBC.