For those who are interested in knowing what the set-up process is like, a recording (from our own production set-up) has been made available here:
https://www.clearpeople.com/blog/microsoft-viva-topics-walk-through-of-set-up
(it shows the steps from picking the license, through to completing the admin centre set-up)
With regards to some of the common questions / comments....
a) Viva Topics requires (currently and as far as I know) a licensed tenant in order to activate the trial or to buy. In other words, trial/dev tenants are not eligible (at least that has been our experience, even as a Preferred Partner in the program).
b) After the initial set-up, it can take a few hours for the actual Viva Topics components to be deployed into the Site Collection (i.e. into to Topics Center)
c) After the Viva Topics components have landed in the Site Collection, it will still take a while (days easily) before any suggested topics appear, but in the meantime, you can start creating Topic Pages manually.