When doing a major upgrade or rebuild, it's a good time to review the content and structure.
This is an opportunity to make adjustments, before the new site is up and running.
High Level Questions
What content types exist?
How many items of each content type exist?
Who are the content authors?
Are there any unused entities/fields?
Does the site use revisions?
Keep users/roles/permissions?
What can be removed (Unused types/fields, unpublished content)?
Approach
A 1:1 migration is the easiest, but making structural changes after the new site is up is harder and you may lose the opportunity.
Checking every possible thing could take you down rabbit hols and take up more time than available.
High level decisions on what to keep, change, or cut are necessary for accurate results.
Start with the larger, common components and discuss how they are used and any changes needed.
Create a checklist for all entity types and bundles that are needed for review.
Consider the entity storage structure, field form/display configuration.
Consider auxiliary things like users and configuration.
Modules
https://www.drupal.org/project/routes_list
List of all available routes.