My two cent take on political strategy is that impeaching too soon would be a disaster. If the Dems had impeached six months ago, the GOP would've given him a pass by now and by the time Nov 2020 rolls around it would be considered old news, water under the bridge, sour grapes, etc. by moderate (read: ignorant) voters. If they time it right, though, with the hearings airing Trump's dirty laundry at the opportune moment, the proceedings still won't eject him but he (and the senators that back him) will do worse in the elections.
Ultimately the responsibility isn't on the Dems for playing politics, it's on the voters for letting the GOP get away with this shit. And arguably the media for failing to convince people that this is a big deal.