I'm back again as I think i've narrowed down the issue.
I grabbed the query for one of the metrics by being in experiment/3 dots/view queries/view_cart. This I plugged into BigQuery directly and removed stuff until it returned anything.
The query (pastebin):
https://hastebin.skyra.pw/lasebifasi.pgsql
I uncommented these two in line 27-28
-- AND experiment_id_param.key = 'experiment_id'
-- AND variation_id_param.key = 'variation_id'
And after that I atleast got something
variation | dimension | users | main_sum | main_sum_squares
___multiple___ | empty | 22726 | 33 | 33.0