Depending on the structure of your base, the workaround that you found might be your best option.
My advice might not quite match your situation as I don’t quite know how you have structured the tables and fields in your base.
For example, suppose you have 5 categories of parts, each in its own table. Each category of parts would have one row per part. Then your [Suppliers] table would have 5 linked-record fields, one for each table of categories. You would also have 5 COUNTALL rollups, one for each category. Then you would SUM the rollups in a formula field.
It sounds like this might be your situation and the workaround that you found.
On the other hand, suppose you have one [Parts] table, with one row for each part. Then you have a [Categories] table with one row for each category. Each part links to one category. Each category links to multiple parts. Finally, you have a [Suppliers] table with one supplier per row that links to multiple categories. In this case, you would have a {Count of Parts} rollup field in the [Categories] table that counts all of the parts in the category. Then in the [Suppliers] table, you would have a rollup that sums the {Count of Parts} to find the total number of parts across all categories.
The second situation assumes that all suppliers have all of the parts in a category, not just some. If suppliers have only some parts within a category, there would have to be a different data structure.
So, you can see that the solution really depends on the structure of the data.
