As far as I know, there is no way to download a GEDCOM of your PARTIAL TREE from Ancestry. I had to find an old copy of Family Tree Maker 2012 from some site, download & install it, then I was able to split my tree.
Using FTM (Family Tree Maker), I am able to export my tree a family at a time and upload to Wikitree. This is the easy, logical way that works for me and keeps it simple. I've already uploaded 4 generations back to Wikitree a family at a time (obviously each generation back, I wind up with double the families to import - for instance, my 5th generation has 16 families just from my father's side).
I originally tried to create a GEDCOM of my entire tree (over 8000 souls), and that proved to be a nightmare for importing, so the above solutions worked for me. Good luck!