Syntex Column In Sharepoint Folder

Try using the list web api directly. Use OData.Feed for exploration (e.g. to confirm shape of complex column types and internal names) and then use Web.Contents for production. Yes, the list endpoint works with document libraries (list is how you access the expanded metadata). See here for main ms learn doc: https://learn.microsoft.com/en-us/sharepoint/dev/sp-add-ins/working-with-lists-and-list-items-with-rest

Note that you can page up to 5k items at once but after that you have to implmenet paging (using List.Generate for paging is best practice).