1000 ppm is plenty! especially at every feed. I wouldn't go over that.
Could be a 9-10 week strain and nothing wrong with letting it go as long as possible. Sunshine mix is an inert medium so I wouldn't worry about a 10 day flush personally. 3-5 days (2 irrigations of pure water) is more than enough for soilless (inert) medium imo. I'd give it whatever additives/bud boosters you want with the next irrigations and then maybe reevaluate where you're at. If you have a x60 magnification loupe you could take a peek at the trichomes and see if any are ambering. But overall, you're getting close. You should see the immature buds people post on here thinking they are done! Yours are at least smokeable at this point already.
So, Well done!