Indesig 2024 Data Merge start position issue

The short answer is Data Merge can't do that, but that doesn't necessarily mean you can't use Data Merge to start and then do other things to your merged data to reconfigure it.

I used to do an annual directory that was basically a Data Merge project that had listing of various lengths, so it was a single record per page, then thread the listings together, copy the threaded text and reflow into a new document.

Ten years ago there was a post on this forum about deleting blank lines and having paragraph styles change in a multiple record merge (Multiple record data merge into paragraph style... | Adobe Community ). It is quite long, but it has some useful information, including scripts for removing blank fields (and discussion of why it is better to do that and then use find change  than use Data Merges remove blank lines) and a script to stitch frames together if the first paragraph is a sequential number, which allowed me to automate my directory threading. I've done a quick search to see if I could find a similar script that doesn't require numbers, but had no luck. My recollection is that I added a field to my merge template for a line number which was added to my sorted records in Excel, put that into its own paragraph with a unique style as the first line in my template frame, then ran a find/change to remove that paragraph after stitching all the individal pages into one threaded story. you can do the same, or perhaps someone else has a script that would work without numbers.

https://www.rorohiko.com/wordpress/indesign-downloads/textstitch/ is a possibility here, I think.