I don't blame you sticking with the default IPB SEO used for URL structure. When I had IPB3 installed I found that google had no issues at all indexing the pages added the same day or the very next. I really do feel that IPB doesn't need any other SEO add-on at all from what I saw on my own forum at the time and how fast Google was indexing the pages.
What I will say though, is I had the forum installed in the ROOT (not a sub-forum folder in it). I think that' the best way to go when installing IPB3, just make sure to edit the robots.txt file to reflect that though, as it's been created by IPB to use a sub folder called "forum".
Quite interestingly also. Because I had the forum installed in the ROOT. All dead links in Google lead to the forums own 404 page. But when I tested the forum in a sub-folder. Instead it showed the server host 404 page instead of the IPB3 one. So that might be worth noting also why it's better to install IPB3 in the ROOT.