“Kick in the ass” is actually a phrase that I might use (native speaker of Canadian English). Sorry, I haven’t found any references (yet).
but I might use it as follows “my boss gave me a kick in the ass that got me started on the project that I was procrastinating about, and it turned out to be easier than I thought“.
Or “getting out of breath when I ran up the stairs was a kick in the ass that got me serious about exercising”
However, a “kick in the ass” can be both positive and negative, as so many things are in English. Most of the definitions that I have found emphasize the negative aspects, like “my boss threatened to fire me, which was a kick in the ass“. But that in itself might inspire you to start working harder. Tough love.
Also not to be confused with “kick ass“ or “kick butt”.
Less colloquially, you already mentioned “push”. E.g. “my wife pushed me to start the project of cleaning out the garage”..
More: “trigger”, “prompt”.
“nagged” as you “my wife magged me until I started working on the project”
IMHO terms like “boost” are not necessarily a thing that start you on a project, although they might help you along the way.
“Pushed me off the cliff” or “pushed me into the water”
E.g. “my friend pushed me off the cliff into the ocean, where it was sink or swim. Fortunately I swam… and that’s how I got started on my career.”
Riffing off one of the other answers “I lit a fire under his ass, and he finally started working on the project”.