I think Chuck Schumer got it right when he was campaigning while President Clinton’s impeachment process was percolating along.
A Senate trial over impeachment is not closely parallel to trials in the court; Senators are not impartial jurors.
Given the split along party lines for impeachment of Trump, the idea that impartiality exists on either side is not very supportable. Regardless, the notion that anyone would be “impartial” after hearing all the public testimony and debate is silly. Surely by this point in time, every Senator will have formed a pretty strong opinion. It’s not as if they have been sequestered off somewhere while the first part of the process takes place.
Part of the delay may be to let Trump provide more reasons for impeachment. We’ve already seen bribery attempts [Trump gifting campaign funds to Republican Senators in battleground states for the expressed purpose of swaying/solidifying their votes]. Release publically of the whistleblower name is another reason. Trump is quite capable of hanging himself out to dry.
I am not in any way under the impression that current senators masquerading as principled Republicans will suddenly grow a spine, develop a sense of morals, and/or put country over cult.