Way to find cheapest combination for multicity tickets?

I'm trying to book flights from Toronto -> Paris/Brussels, staying in the north of France for 2-6 weeks, and then going to Seattle. I have to leave Toronto between after 5p on 25 April and 01 May however my trip to Seattle is quite flexible.

While Google Flights and Kayak have great tools for viewing multiple destinations (Paris and Brussels) and multiple dates +/- 2 days, I'm struggling to find similar tools for their multicity options.

Does anyone have any suggestions?