Think Boris would have liked to have got the deal passed before a GE, which would wipe out Lib Dems and Brexit Party, but practically, he can't get it passed without amendments that would prevent it getting passed, so can see why he went for GE instead. But it does get a GE as a pseudo-referendum which is what he wanted--if it was based on things like their past performance, doubt they would want a GE.
I also think that getting a deal before calling the GE will take the rug out from the Brexit Party, now Farage is forced from a Brexit vs. Remain battle into a No Deal vs. Deal battle, and I think he can only win that with a small minority of extremists. Further, due to the massive headache that hung parliaments, coalition parliaments, have created, I doubt any voter wants an election to return another of those, only to lead to another GE sooner rather than later. Due to this, I expect tactical voting to severely affect the smaller parties, in particular TBP and Lib Dems, with their prior support not translating into election votes and seats. In terms of splitting of the vote, I can see Lib Dems more effectively splitting the Remain vote and adversely affecting Labour, than TBP affecting the Tories, in part because of the mishmash policies of Labour, because of Corbyn himself, and Lib Dems going unequivocally Remain and Revoke. For all these reasons, plus polling trends, I predict a Conversative Landslide majority win.