My amateur opinion:
Add another build week between week 16 and 17. It can be less volume or intensity than week 17. For example, if week 17 is similar to week 15, then make the new week similar to week 14; if week 17 is more than week 15 than make the new week like week 15. If you’re feeling strong (honestly), you could also just add another build week after week 18, that’s more of a build. I think as long as weeks 19 and 20 are stable, and you take a rest week every 3-4 weeks you’ll be fine. Also, I like to put my longest long run about 3 weeks before the race, some do it more like 2 weeks–so this depends somewhat on when day each week of the training plan starts for you, etc.