Not really an answer to your question but I recently imported a GEDCOM which I made from my Ancestry tree, no where near as big as yours is though :). From my experience it is a great thing to be able to see your ancestors all attached as you click on one but every profile you add via the gedcom will need a clean up, because a lot of gobbledy gook is created in the process. I have started on cleaning up mine and in the process decided to re-source the information from Familysearch purely because my decendants in the future may not wish to purchase a membership from Ancestry and therefore won't be able to see the relevant information in situ. By doing this I have managed to confirm my original findings or in some respects correct them. Don't worry about making mistakes, we all do and here at wikitree I have found there are so many helpful people that will assist in correcting errors and break down the proverbial brick walls that may arise. One wikitreer has just done that for me. I have actually found more information to add into some profiles than what I originally had. From a personal perspective I think I have now found the most comfortable way for me to work.
Whichever way you choose the most important thing is to enjoy what you are doing
Welcome to wikitree, happy reearching