I do now recall some issues around
Cannot deserialize the current JSON array (e.g. [1,2,3])
Nested content v13 to Block content v16 and migrate data - Umbraco community forum
That’s probably a bit early for you…
But have you updated to all the latest Umbraco v13.6.1 and usync v13.3.2 to make sure all schemas are up to date and in sync?
beyond that you can try an track this down if I remember correctly, you’ll have a v9 usync folder with a content folder.. find the config for one of these pages that has the nested content, that’s failing.. then temp move all the other content configs to another folder. then you can rerun the migration and it will only try and do that one config and you can see by manually updating the xml and rerunning the migration what property is causing the error.
But manual and long winded.. but could at least get you to what is causing the error 
And just a final thought.. you are running post usync migration, import settings and import content? as the screen shot has
