One does not need proof to bring someone to trial. Only sufficient evidence that shows culpability. Then it becomes the objective of the trial itself to show proof or show that insufficient proof exists.
Impeachment is similar to indictment. Only the House can impeach for any reason they see fit, even if there’s no evidence. However, without evidence of wrongdoing at all, getting a conviction out of the Senate would be a tough job.
I don’t think we are at the stage of articles of impeachment yet. We still need evidence such as the Comey memo, testimony from witnesses (which would include the members of his campaign team on the collusion issue and members of his staff on the classified information, Comey firing, and meddling in FBI investigations issues). Testimony under oath, of course, and with penalties for perjury clearly explained.
This is how Watergate worked. House Judiciary hearings, evidence collection, lots of subpoenas, and eventually articles of impeachment. However, if the Republicans make no movement in this direction, look for a very angry America to turn the House over to the Democrats next year. Of course, if that happens, the Democrats will move on this. And the GOP will be in tatters for awhile, as they were in the wake of Watergate.