As you said it is circumstantial, it could be that that there were more exchanges of (truly) sensitive information between Manafort and Kilimnik (they used encrypted communication) and that Kilimnik does in fact have GRU access (or is GRU himself), or everything could have been orchestrated by Putin himself to put Trump into power, I don't know that, but that needs to proven.
But doesn't X amount circumstantial evidence together prove collusion? Not necessarily, you still need to weigh each circumstantial evidence.
Lets take the WikiLeaks example, assume that it is 100% true that it was the GRU that was feeding WikiLeaks with the stolen documents. Trump campaign benefited from those leaks and Trump encouraged WikiLeaks to do more, does this prove Russian collusion? No. There is no proven coordination between the Trump Campaign and GRU via WikiLeaks. GRU could have played on these events during the 2016 election and used that to benefit the candidate they would prefer, assuming Trump. Putin is on the record for criticizing the US interventionist policies, policies that Clinton represents, and they could have used this opportunity to help the Trump campaign and the Trump campaign gladly accepted this opportunity to get an advantage, but I would say that should be viewed as interference from GRU, not collusion with the campaign.
That is why the claim from the Senate report that "Kilimnik may have been connected the GRU's hack" is interesting, but the report doesn't provide any evidence only speculation on redacted material. That is of course impossible to asses the truthiness of.