That‘s the route I would have taken if I did not have a Beast. If you believe you can DIY why not. If you follow the application process you can wait on the brackets until they are readily available. My light bar was on solid. Brackets give it an extra margin of safety.
There are several in addition to the adhesive. The service manual is your best guide for that process: https://service.tesla.com/docs/Cybertruck/ServiceManual/en-us/GUID-D0FE9E8D-AABD-4DBB-9334-92FDA16FACD0.html
Many of the new CTs off gas from the interior material. Have you cleaned the inside of the windshield well? Good video on YouTube from out of spec for that purpose.
I would address this with customer service. The light bar kit includes everthing enumerated in your invoice. Some knuckle head is charging extra for parts. The service advisor needs oversight and further training.