If I understand you correctly, you want to do a series of GEDCOM uploads to the same Ancestry Family Tree, not causing a new AFT as it does now with each upload. The only way this can work is for the AFT to be first emptied or for it to be deleted after upload to a new AFT that is then reassigned the former AFT's identity. I agree it would not be 'sync''; rather, just an online representation of your RootsMagic database limited to those things in GEDCOM that both RM and Ancestry support. It would be a publishing tool that allows you to republish an AFT as many times as you want and accessible at the same URL.
It would not be very useful for researching because your sources in RootsMagic will be external to the Ancestry databases, even if they are cited. Ancestry Hints will be generated for many of those same sources.
Any changes you make on the AFT cannot be downloaded to your RM database; that would be 'syncing'. Unless you manually edit the RM database, those online changes will be overwritten on the next upload.
So what you want and what does not jeopardize FTM is the option of replacing an AFT with a new upload. Purely a publishing tool.
That is pretty close to what I was thinking of, but from what you say, I may be mistaken about how the sources are attached to the database in Ancestry. I was assuming they were attached to the database # and the individual # so that unless the document or the individual were deleted the source documents would stay attached. Following that thought, I assumed that the initial gedcom upload would get those identifiers set so by re-uploading corrections it would leave those identifiers intact and just overwrite the data. Please forgive the syntax, I have not programmed in several years and have not kept up as I should with some of the newer languages.
Anyway, my thoughts were just to be able to update corrections to data that way, rather than go online for each change. I uploaded my database just so I could play around with it and check out any new info on my "brick walls" It found almost all of my sources and some more which was really helpful so I went ahead and attached them just to see how it all works. I then went back in to the source document so I could save it to my computer and then I would manually imput the new info from the source document into Roots Magic. Although I would love to be able to keep the attached documents so family and others could see it online, from what you say this will not be possible if I were able to upload corrections via a gedcom. So it would be a trade off. As I see it, I have 3 options:
1. I could put the database up there, make it private and just use it for research. I started doing genealogy before PC's and by the time the first gen program came out I had a couple thousand people. I used the first version of PAF and it did not have a separate section for sources - they were put in the note section preceded by an exclamation mark. By the time, Family Origins came out, I had too many people to change them all over to the source section, so for all these many years my sources are still in the notes. As you say when I get hints, I get a lot of my old sources, but surprisingly many new ones cause ancestry keeps getting more and more documents. The one advantage is by having your tree online, it save hours filling out search forms and the hints are wonderful. After a few months I would delete that database and upload another gedcom and continue searching. Downside: no point in having others look at it cause no need to attach photos, etc.
2. I could export gedcom from RM to FTM, then use FTM to communicate with Ancestry. This way I could update and keep attachments in place. Still not interested in syncing down to my computer, but if it did it would only affect the gedcom in FTM, not my original database. I would still make all my changes manually to my original database in RM and save documents to my computer. After making changes/corrections over a period of time, I could export a new gedcom from RM into FTM and sync those changes up to Ancestry.
3. Switch to FTM, which I will NEVER do! I have disliked FTM since it's owners bought out Family Origins and trashed it cause they could not take the competition. I do not know even if Ancestry owned it back then, but I did not like that type of business practice.
Anyway, I would value your opinion on all this! Thanks for listening!