Mailstores free home version works pretty well for this. You can backup your IMAP account in it, and then it has the option to export that archive to another IMAP account. It does retain folder/label structure (although it places them under a parent "Mailstore" label, but that's easy to fix after).

It will also read EML, MSG, PST, and MBOX formats, and connect to Outlook and Thunderbird.

https://www.mailstore.com/en/products/mailstore-home/