Hello,

I'm tempted to get Humankind on Stadia and PC but it will be quite expensive to buy all the DLC twice as well.

With some games on Stadia like Elder Scrolls Online, the DLC that you own are shared across the account so, you only need to buy the base game on each platform.

Is it possible if something like this will be implemented with Humankind as well?

Highly unlikely. ESO sells all its DLCs via in-game store, so they are tied to Bethesda account. Even if you purchase, say, on Steam. Amplitude sales its DLCs via 3rd party stores, and I doubt they will push G2G integration that far.