If you have stock movements associated to the same product-site happening concurrently then you have a risk of having a lock error. So my advise, if possible, is to have 1 pool of records (1 file if using flat file) per stock site. If you have different stock sites, there should be no reason for having a lock error on STOCK table (except for lock escalation - in that case you can deactivate this in the database). But you have no way to change the update record lock logic -> if 2 concurrent transactions try to modify the same STOCK record you must forbid one to prevent data corruption.
In your case, you just need to re-run the records in error to normally complete the import process. I understand this reprocessing can be manually but this is maybe the best place to invest extra effort.
I hope this helps.

Thanks for this Julien, much appreciated.

Thanks for this Julien, much appreciated.