As for bricking your device: It will not hard brick, it may only soft-brick[0], as in, the display will not update on the rM2, because the software driving the screen needs to be updated to support the new OS. You can still SSH in and revert the changes and then downgrade. If you were silly enough not to write down your SSH password, or set up an SSH key and can't get into SSH, you can still recover your device[3] on your own.
0. https://remarkable.guide/faqs.html#can-i-install-toltec-befo... 1. https://remarkable.guide/devel/toolchains.html 2. https://remarkable.guide/guide/access/ssh.html 3. https://remarkable.guide/tech/recovery.html 4. https://github.com/sandsmark/recrossable