Add IndexedProperties on ListItem level
Currently you can only add indexed properties on Site, Web, List or Folder level - I suggest you to add the same on ListItem level.
If you want to enrich items with additional metadata you have to use list columns or content enrichment (not available with SPO or with hybrid cloud search service).
Another option would be to add "normal" columns to the items (and maybe hide them from editview) and rely on the normal indexing - for a large scenario this is simply not possible.
Indexing of the propertybag would greatly simplify this.