Wass1947
Hello, you've made the right choice moving to the Pixel 8 Pro. Awesome security form MTE, and I know I'm really loving mine!
If you are having troubles with unlocking the bootloader with the web based installer, perhaps this will help; try installing Platform Tools, the set of tools that do things like unlock the bootloader, relock it and allow flashing of things like GrapheneOS to your phone! Here is how you install the latest version incredibly easily, your lucky you are running Ubuntu like I am, they have an installer in apt that fetches the latest. Type into any terminal:
sudo apt install: google-android-platform-tools-installer
Then, after connecting your phone to the laptop (preferably with the USB C cable that came with the Pixel 8 Pro), type:
fastboot flashing unlock
You should get the message on your phone saying "do not unlock the bootloader." Just use the volume buttons to change it to "unlock the bootloader" and then you can go back to the web installer!