I'm very attached to owning my games and supporting directly the developer so I would highly prefer buying the game on itch.io, and I like having access to older versions, but I don't want to miss out on Steam workshop content.

Is there a way to get Workshop content with the itch.io game ?

Unfortunately we have decided to stop offering Steam keys for our games going forward due to the way Valve uses the external keys system to strong-arm developers. If workshop content is very important to you, I recommend you get the game on Steam. We're committed to keep older major versions available on Steam through beta codes which you can find in the changelog thread on the Steam forums.