using TB to get all yahoo emails into gmx

First you need to get all of your mails out of Yahoo. you might try using the importexport tools addon for that saving the results as MBOX files. The same addon will attempt to import mail into a local folders account and it will then be up to you to figure how few you can upload at a time into your GMX account as GMX do not offer a direct email import option. Nor does Yahoo offer an email export option. Thunderbird is a mail client and some cross account mail copies will work well. Bulk is not something anyone even considered when the IMAP specification was written in the 1990s nor was the sheer size and number of email folk now consider normal. Hence Yahoo having issues synchronizing large IMAP mailboxes locally and reducing storage to cover the problem. A mail client is not some file system that copies bulk mails from one account to another so while you will have some success on limited numbers. Dropping an entire Yahoo 15Gb mail account into another is almost certainly doomed before you start. Local export is perhaps the best result you can hope for without spending a lot of money or time and then maintaining you own local backups. Transferring a month or two might work, but you will never really know if you got everything copied and there is no audit trail.