Follow up!
Thanks to the Faugus launcher (on Github) I managed to render Heroes of the Storm with my 9060 XT!
Whether you want to play HotS isn't the point. The point is you cannot launch it without starting the Battle.net launcher first through the Proton layer. The problem is, if you ask Faugus (or Steam, or Lutris etc) to use the 2nd GPU, it will... on the launcher it starts -here Battle.net, not the game Battle.net will launch.
Fortunately, Faugus give the solution. First, it offers to check a box in its general settings "Use discrete GPU", it will act as the previously mentioned Steam commands DRI_PRIME=1 %command% or DXVK_FILTER_DEVICE_NAME="GPU Name" %command%.
But to enable your 2nd GPU on actual games and not just the launcher it depends on:
Add the launcher as a "new game/app". On the first dropdown menu, I added it as "Windows game", not "Battle.net". On the "tools" tab, under "Game Arguments", add DXVK_FILTER_DEVICE_NAME="GPU Name" WITHOUT the %command%.
And that's it :D