Recommend a working indexing service
Zeto
Advise a working indexing service with adequate prices. To make sure it works
Zeto
Comments
Using a single tier of gsa links pointed at the links you want to index seems to be the most effective and affordable option.
If they are using GSC as they say they must submit URLs in bulk, there.
I also use the sitemap method to get my links crawled, but I ping different google regions with a sitemap of my backlinks.