I'm sorry you've had bad news.
I won't get into the continuing a pregnancy stuff as, like you say, you've made up your mind.
I think your next move is probably a cvs for a diagnostic result. It is unlikely the harmony is wrong and obviously you are entitled to a termination even if this baby doesn't have T21, however, personally I think a proper diagnosis is better. It will more than likely be positive but there is a tiny change the harmony is a false positive.
I have had a tfmr. Our dd2 had T18. However, my termination was at 22 weeks so a bit more involved in that it is a longer process and you are induced for labour and labour naturally. The labour part involves a tablet taken to suppress your hormones and then back 48 hours later for induction drugs.
My labour with dd2 took around 7 hours from the first pessary to her birth. I had gas and air and morphine for around 3 hours. However, like I say, she was that bit bigger.
I have just been through the same (ish) process at 14 weeks (although it was a late miscarriage of what we thought was a healthy baby rather than a termination, the process is still the same before 20 weeks though) I had medical management, same medication except I only needed one dose of induction drugs and ds arrived within 4 hours. I had gas and air for half an hour or so at the end. I probably could have dealt with the pain but I figure why the hell should I.
Both our babies have memory boxes and photos taken. We opted to see both of them afterwards and spend so,e time with them. This is offered wether you have had a termination or a mc. The choice is up to you. We also had a memorial service and a plaque in the children part of the crematorium. We will be doing the same for ds too.
I absolutely understand the the whole 'seeing and remembering' the baby might not be helpful for you but what I'm trying to say is that having a termination doesn't mean you don't have those options if you would think they will help you.
I am more than happy to answer any questions you have. I know in my trust the cut off for survival management is 13 weeks (I missed it by a day as ds died at 13+1. However, I would have opted for medical anyway as I wanted to see him)
Again 💐 I am sorry it's such a shit time for you.