If Trump were a miserable president things would be easier for Pelosi.
Trump is a miserable president. However, he's a miserable president who inherited an economy that was not only growing, but which had been set up for a prolonged period of strengthening growth, by a prior period of massive deleveraging and fiscal improvement. Obama left a country where the deficit had fallen by half compared to what he inherited, and maybe more importantly, a country in which private individuals had spent years reducing debt as a share of GDP. Household debt had been 98.5% of GDP at the end of the Bush Catastrophe, but by the end of 2016, it was down to 80.1%, which was the lowest it had ever been since the Federal Reserve started tracking that statistic. That took average debt service payments down from nearly 13% of disposable income to around 10% -- which was somewhere we hadn't been since before the orgy of private borrowing in the mid-80s. We were not only growing, but situated such that consumers and businesses could borrow (and borrow cheaply, thanks to low interest rates) to keep that growth going. Put on top of that a violently sudden increase in federal budget deficits (which just set an all-time quarterly high), and you've got a whole lot of stimulus to create short-term prosperity. That has really helped immunize Trump, despite how miserable a president he's been.
Trump has yet to make a serious foreign policy mistake—much less, get us into a war.
Trump has made a long series of very serious foreign policy mistakes, resulting in an absolute collapse of America's approval ratings around the world -- especially among the other leading nations whose cooperation we need to address global problems like terrorism, financial crises, climate change, etc. However, as you say, he hasn't yet gotten us into any major war (though the US has been committing various acts of war all around the world, in the small-scale). That lack of what I'd call a "charismatic" foreign policy mistake has, indeed, made it harder to go after him. It's not like people have been watching a steady stream of bodies coming home from some unforced-error like Iraq.
The other problem is a Senate trial is just that—it’s a trial. You never know what’s going to come out in a trial and Pelosi knows that there’s no guarantee that everything that comes out is going to be bad for Trump. It would very likely go the other way since Mullet’s report is public knowledge and things about the Russian investigation are just starting to leak out.
I doubt that plays much on her mind. Throughout this process, the more we've learned, the worse Trump looks. And, at this point, he looks dirty as hell. There's every reason to expect that will continue to be the case -- that he'll look worse and worse. And if he's put under oath and asked questions, it's virtually guaranteed he'll perjure himself. Rather, I think what Pelosi realizes is that it won't matter what comes out. The trial could result in a giant pile of smoking guns, and the Republicans will all vote to acquit. That's just in their nature. And the Fox News crowd will be painstakingly sheltered from relevations of Trump's wrongdoing, while fed a steady diet of out-of-context factoid and soundbites to convince them that the trial cleared Trump's name... the same way those walking afterbirths came away from the Mueller Report convinced it cleared Trump. So, what would the point be? Trump won't be removed from office, the informed people are already going to vote against Trump in the next election, the imbeciles are going to vote for him regardless, and the sideline sitters might just be annoyed enough by having impeachment coverage interrupting their boob tube swill that they break Trump's way, too. There's little upside and significant risk of down-side, from a pure partisan politics perspective.
Which brings us to the proverbial Elephant in the Room. The investigation is under investigation by a determined AG and by his appointed prosecutor Durham. This isn’t a congressional dog and pony show investigation any more.
The AG's dog and pony investigation is a joke. There's no sign of any wrongdoing to investigate, but he figures the dummies need to be told that it's actually the investigators who were in the wrong, so he figures if he blows enough smoke they'll assume there must have been a fire.