Harmony IS statistical, but it's a 1 in 10,000 chance of a false negative. That's good enough for the doctors at the Fetal Medicine Centre in London who are at the forefront of research in that area. It's not like NT screening which looks at hormonal markers and the NT fold, all of which can be outside textbook for completely benign reasons. It actually looks at the free floating fetal DNA in your blood.
If you'd had 1 in 10,000 for your NT screening, would you have been happy? If not then of course consider an amnio if you absolutely have to know beyond a doubt, as 1 in 10,000 is the best Harmony will give you. The ONLY other result it can give besides 1 in 10,000 is 1 in 2, which means baby probably has Down Syndrome (an amnio is then recommended for confirmation) - there are no other results in between. High risk, or low risk, that's it, unlike the NT screening which gives you all those numbers.
Even an amnio is prone to errors in results, mostly human errors as opposed to the method being dubious, but errors nonetheless. It's very rare though, but then so is a false negative with Harmony.
I had a 1 in 5 result at my NT screening and Harmony says I am low risk. I'm trusting Harmony's result as does the FMC where I had it, but I appreciate not everybody will. Last pregnancy I was 1 in 34, non-invasive testing was not available then, and I did not want to risk the baby so I waited it out, baby has typical chromosomes. It is rare, but I have seen friends (plus many more on forums) lose babies to amnio and CVS and I could not live with the guilt myself - but it's an extremely personal choice based on your own tolerance of risk/conscience. I am very grateful for the new non invasive options we have now and they will save a lot of lives as most people and doctors are happy with the results they give.
Good luck with your decision! As others have said, 99% chance of a healthy baby.. that's far far better odds than making it out the first trimester (75-80%) which you've already done!