I usually put it in after I blend my lambics into an actual "gueuze". Or, into another vessel to make sure it's done fermenting before I add a ton of sugar back to it. The point being that fermentation has stopped. You could pasteurize as well, but yuck.
And strawberries, it takes too many for me to get anything from them so I stopped using them years ago. If I want strawberry sour, I'll give the Hannsens more of my money. I've given them enough over the years as theirs is my favorite, and I think the best.
But...tiny wild strawberries are way better. And I've seen some Japanese strawberries that are packed with flavor. And go for a premium, like buying a new 80" TV type of premium.