A backlink does nothing until Google indexes the page it sits on. The pipeline we use for backlink indexing campaigns:
1) clean and dedupe the URL list;
2) submit in controlled batches (standard or instant);
3) verify separately - split results into Indexed / Not indexed / unresolved;
4) export and reprocess only the failures.
Choose a service with an honest index rate (no fake 100%), no Search Console or domain-ownership requirement, and verification decoupled from submission.