Best way to assess potential collateral damage from removing fields.

Your best bet if you want a good visualization is probably to build the relationships out in something like draw.io (free option) so you can see how they are used. In the field properties of a lookup field they also have a dependency diagram, which will show you how the field is used a little deeper than the Usage tab does.

You can also create a full copy of the app and all the data within it and then do your changes in the app copy to figure out what needs to be done. Write down your process once you have figured out what works and then build it in your main app and use the copy as a reference.

When I am looking into migrating from one set of fields to another I go through each field and look at the usage and plot out exactly what changes need to be made for the new field(s) to work. You never find everything but doing this you can plan for most of it and handle the rest as it comes.