I'm using https://add-my-url.com/ services, they have the best price for a backlink per UDS$ and the indexing rate is 50-60%, but they don't have a drip-drop option yet... Omega indexer has the same indexing rate but after the last google update in July you should keep your eye on the GSC backlinks list and disavow toxic omega indexers from your console or you will be penalised. Also two weeks ago I started testing https://elitelinkindexer.com/, their rate is 30-40%. In the future, I plan to use both, elite and add.my.url