Thanks for the info, I'll try that out. I tried the epub-txt-rename method and although it works, it unfortunately (as you've mentioned) swallows the cover art on my Kindle Oasis. I'll try the Calibre method you've described next, hoping that it maybe is reliable for my files. Would be the most happiest to use Amazon Kindlegen, for sure, but I'd definitely need a batch conversion possibility for that. Tried to find an Apple Script (I am on a Mac), but with no luck. An instruction I found using "Sigil" together with KindlenGen sadly exceeds my competences – I simply don't understand how to use this together. Installing a Calibre plugin or using an already written Apple KindleGen script is my limit ...
If anybody out there has a hint for an Apple script or similar which I could run with a folder full of epubs/mobis for batch convert, I would be glad if he/she could let me know ...
Thanks a lot and best from Germany!