Nice project !!
Your next step "to lock the TEENSY without bricking it" is very easy !! From the Arduino 1.8.19 IDE, you simply select "Tools > Teensy 4 Security". From the popup dialog, click the "Generate Key" button. This step will result in creating your private key (the dialog shows where it will be stored). Pay particular attention to the warning that you MUST store your key securely (also, make sure that you have a backup somewhere else, also stored securely . . . if you lose this key, you completely lose the ability to create the locked hex file required (re)program any Teensies locked with this key). This key generation is a one-time necessity (although you could potentially manage multiple private keys, doing so requires very careful management to make sure that you always use the right key from your collection that matches the key used to lock a given Teensy in order to create the new locked hex file).
Once you have your private key, you can then click the "Fuse Write Sketch" button to open a sketch that allows you to setup/prepare a particular Teensy to be locked. Following that, clicking the "Lock Security Sketch" opens a final sketch that allows you to actually set the Teensy as locked. This is a non-reversible step: once a Teensy is locked, it can only be reprogrammed using an eHex file that has been created using your matching private key. The required eHex file is automatically generated by the build process if/when a private key is found in the right place . . . you don't have to do anything special and/or different when building the sketch that you're protecting. Your private key is only required to create the eHex file, it is not required to actually (re)program the locked Teensy.
Once you have completed these preparation steps for using a lockable Teensy, either the 1.8.x Arduino IDE or the newer 2.x.x Arduino IDE can be used to create your sketches and/or to update/load your locked Teensies. There are other 3rd-party development tools. I don't use any of these, so you'll have to look for specifics yourself if you are interested in using anything other than the Arduino IDE tools.
Despite what you might read, there is absolutely *NO* chance of bricking a lockable Teensy, if all of the steps are properly performed. Of course, you will lose the ability to build the proper eHex file needed to (re)program any Teensies locked with a particular private key if you lose that key. In this case, the Teensy is not bricked, but there is absolutely no way for that particular Teensy to be (re)programmed without using the same key that it was initially locked with to build the eHex file. That's part of the "security" of the lockable Teensy, and that's a GOOD thing !!
NOTE: I personally found it best to cycle power on the Teensy after each step of the initial locking process (as described/provided in the "Teensy 4 Security" dialog). I have been using lockable (and locked) Teensies for quite some time, and I have never had any problem after locking them. I predict that your experience with locked Teensies will be likewise just as successful.
You can find more details on the additional capabilities and protections provided by the lockable Teensy at the following link:
https://www.pjrc.com/store/teensy41.html#codesecurity.
Good luck & have fun !!
Mark J Culross
KD5RXT
