Can you split your GEDCOM at MyHeritage? The max upload here is 5000. If you can't split it at MyHeritage, you may have to find a program into which you can put your GEDCOM which will split it for you.
Most folks who work from GEDCOMs here say it's best for the GEDCOM you upload to contain 500 or fewer individuals.
If you'll click on "My WikiTree" tab at the top right of this page, you will see GEDCOMs in the drop-down menu. Follow the instructions to upload your GEDCOM. Once uploaded, you then have to go through a process called "GEDCompare" where you compare every person in your GEDCOM to potential matches which are already on WikiTree. (This is why a smaller file is better.) Once you've either accepted or rejected every potential match for every person, then you will get the "ADD" buttons to start adding the unmatched people in your GEDCOM one at a time to WikiTree. GEDCOM-created profiles will contain some "junk" which has to be cleaned up and your sources may not be in good format, so you will have to work on cleaning that up for each profile, as well.
It's complex, but it ensures that you do not create duplicate profiles for individuals already on WikiTree.