One thing people miss is crawl depth. If your backlink sits on page 9 of some profile/blog/category with zero internal links, Google may never touch it even if the domain is decent.
I usually test a batch first: check if the linking page itself is indexed, then throw 2-3 cheap tier links at it from pages that actually get crawled fast. Not random indexer blasts, more like fresh posts, RSS, social bookmark, maybe a web 2.0 that already has some activity. If it still doesnt show after couple weeks, I just mark that source as dead and stop wasting links there.
Also dont judge by ahrefs/semrush only, they lag and miss alot. Use site: check + GSC on your own side for movement. Google is ignoring junk harder now, so filtering bad link sources saves more money than trying to force index everything.