Let's get it clear. In order to transfer your data to Reunion you need to create a gedcom. There is no other way to do it.
I don't use Public Member Trees so don't know whether if you are successful in uploading your file you might be able to export one from them.
The only other ways I can suggest to save your data is dependent on you still having either the Windows machine you used to use and the Windows version of FTM, or access to one.
Worst case scenario you can, provided you kept the file, try to export a gedcom under Windows from you last Windows file. If successful you will lose whatever changes you have made since you switched to FTMM.
I have already told you about converting the Mac file to a Windows version and then trying to export a gedcom.
Otherwise I fear you will have to start again!
I remain astounded by the fact that you were able to use either the Windows or Mac versions with such an apparently large file.