One of the biggest lessons we learned with internal linking is that most sites already have enough content; it's just not connected in a way Google understands. On one project, we noticed several posts ranking on page two for related terms, but none of them were clearly supporting each other.
Instead of adding more content, we picked one page to act as the hub and rewrote its structure to clearly answer the main topic. Then we linked to it from 6-8 supporting articles using plain, natural anchor text, not exact-match keywords. Each supporting article was also linked to one or two others, so the relationship between topics was obvious.
We tracked changes in Search Console, no fancy tools. Within a few weeks, the hub page moved up the rankings, and we started seeing backlinks come in naturally because people were sharing the main resource, not the smaller posts.
Internal linking works best when it mirrors how a person would logically learn a topic: start broad, then go deeper, instead of trying to force keywords into links.