Hi JTO_SBM,
Thanks for sharing your experience with us.
From your description, I understand you want to create multiple dependent dropdown list by using the UNIQUE and FILTER formula with the Data Validation Toolbox.
When I test from my side, I find that we cannot directly use the formula inside Data Validation Toolbox because it cannot find the unique value from the table.
As workaround, you may create a "transit station" which is used to firstly get unique value from the table, and then create data validation based on the value from the "transit station".
You can find the example file from this shared link: https://1drv.ms/x/s!AqyUzW7T2xzFlheZDi_pVaNzuTS-?e=JKScX0
Merry Christmas!
Best Regards,
Tina