Thanks for this advice. I ended up attaching our logic to the DiscoveryCategoriesRoute, via the didTransition action. This appears to be working well, at least locally. I’ll be testing in staging soon and will update this topic if we encounter any other issues.
I do want to point out for anyone who might be struggling with a similar timing issue: DiscoveryCategoriesRoute only works because our default landing page is the categories page. If we changed it to Topics, New or anything other than Categories, the modal wouldn’t be scheduled to run unless the user navigated to the Categories page-- which might be a bit confusing. 
I’d imagine the same kind of thing could be done for some other route or component in that case.
At any rate, this will work in our use case, and thanks again for the help.