Visualisation tooling IMO is the way to reason with this.

You don't look at the code, but use tooling to create a chart of the calls, data models etc. Then you can look at that and see the complexity.

...and we already had these tools in the early 2000s, when (can't remember which) no-code fad was running about. You know the ones where you just draw the boxes and lines and poof the code is generated =) There were also tools that did the reverse.