Ok, I just quickly wrote some code to test the @Paras’ suggestion to follow the godoc upsert JSON example did the trick.
I also feel @MichelDiz was right to highlight the GraphQL vars do not work with upserts at the moment – I really hope they will do, soon.
I’d like to accept both answers as solutions because I believe both are correct. I shall leave you two to decide who gets the green tick 
Either way, thanks to both of you for pointing me in the right direction!