Permanently Disable Bluetooth on MacMini 2019
The most foolproof way of doing this is simply to disconnect the physical antennas. Then no matter what kind of firmware updates, operating system updates, reboots, or whatever happens - it's not going to enable itself again.

You can follow the guide for disassembling at from iFixIt:

https://www.ifixit.com/Teardown/Mac+mini+Late+2018+Teardown/115210

In step 7, you'll see that the antenna connectors are located on the main board just below the speaker assembly.



By the way: From the sound of your question, I could guess that you're wanting to place the Mac Mini in a data center that forbids the use of wireless devices. In my experience, these places are not so restrictive that you need to actually go through with this. Their rules just mean that you're not allowed to use WiFi/Bluetooth for actual data transfers in the data centers. It doesn't mean that if the Bluetooth antenna is powered up for 30 seconds during a FileVault boot, that they'll sue you for breach of contract. Perhaps try asking them how seriously you need to take that requirement.

EDIT: You have updated your question to say that you wanted to remove the Bluetooth card, and preferably doing so without voiding warranty. That is definitely not possible.

The "card" is not a card - Bluetooth is implemented as a soldered on module from muRata containing not only Bluetooth radios, but also the WiFi radios.

It seems like your requirements for hardware doesn't really fit a Mac Mini at all - so perhaps you should look at buying a different computer instead?

Or you could consider changing the default Apple software. For example you could run Linux instead of macOS on the Mac Mini - where Bluetooth as far as I know is not supported (and if it is, it is easy to disable). This also would mean that you would no longer be using FileVault 2, and so that part of the problem would go away. At that point you might as well buy different hardware since you haven't got a computer yet.