I have had the same problem of not being able to update from V1.21 to V2.1 on several devices (not just Google Pixel). However, this method has usually worked for me - without data loss - namely first install V2.0 and then update to V2.1 via OTA update.
How to proceed:
Download e-2.0-t-20240508399779-dev-sunfish.zip and recovery-IMG-e-2.0-t-20240508399779-dev-sunfish.zip
Unzip recovery-IMG-e-2.0-t-20240508399779-dev-sunfish.zip
are included - flash this two files via …
fastboot flash dtbo dtbo-e-2.0-t-20240508399779-dev-sunfish.img
fastboot flash boot recovery-e-2.0-t-20240508399779-dev-sunfish.img
Now reboot into e-recovery V2.0 to verify the installation
(Note: do not delete anything, so no factory reset either)
adb sideload e-2.0-t-20240508399779-dev-sunfish.zip
Reboot to system