Can't you access you're device with a second one to back up all your data and and do a fresh install?
Discussion
Or you could even try to boot from a bootable USB with Linux Mint or Ubuntu to rescue your data
You usually have to toggle some software or even hardware switches to get around secure boot on Chromebooks. In case of the Pixelbook you need a special USB key cable that’s no longer in production, afaik.
I didn't know that. I thought it's possible as with every ordinary laptop.