Jesus, I'm so dumb...while I knew that I could do a meta-data bulk edit, I completely overlooked that the search function works on the device as well.
And that cut the amount of clicks down by a huge margin...
1. start calibre, killed the fiction unread tag in the library
2. switched to the device, made search for "fiction unread", selected everything from the search result, right click "remove books"...
3. switched to the library mode again, did another search with "fiction" + "ondevice:false", selected all results
4. bulk metadata, add tag "fiction unread"
Result: 150 books showing up in my library as being unread, now I only need to check the quality of the descriptions in the metadata comments field of the 100 old books, convert them and finish with uploading all 150 books back to my device again.
Still curious if I shouldn't switch to using a catalog or maybe some plugins I don't know about (saw there was a reading list plugin somewhere)...