Trump isn't being impeached for alleging the election was stolen. The accusation supporting impeachment is that he incited people to attempt a violent insurrection.
As for a difference in degree between what Democrats say and what Trump has said, you are right, you can't compare the two. The Democrats have been much worse, so I assume you'll be calling for them to be held accountable too. I can show you clips of Democrat politicians (and anti-Trump public figures) saying much worse than anything attributed to Trump. If Trump deserves to be impeached for telling people to "fight" for their country, then what about The Obamas telling Americans "their very lives" were at stake in this election? What about Maxine Waters telling people to accost Trump-supporting Republicans anywhere they find them? What about Pelosi saying she couldn't understand why more people are not out rioting and burning cities? What about Bernie Sanders stating that the Republican stance on health care would kill thousands? Let's not even try to count how many in the media over the last year have dismissed violent protest and assaults, or worse claimed them to be legitimate protest.
Public figures have a duty not to ratchet up the heat irresponsibly. Yes, Trump was stupid to say the things he did, but insurrection? I'm sorry, that's a total gaslighting of the public, and hypocritical to boot.