Most people start with a viral load of around 1 million and if they relapse, they relapse to a similar viral load.
So while people relapsing would pass through undetected, just detected, very detected, it is extremely unlikely that a < 80 at 9 weeks is not also <30, <25, <15, <12, <10 (which are some of the other test sensitivities).
So, yes you should be optimistic. The vast majority of people that relapse are detectable at 4 weeks post-treatment and you are well past that.
Statistically, it is 99.5% certain you are fine. Yes, the 0.5% is annoying but you should be good.