Elgoog Pixel tablet is supported till June 2028. If you can get a used one thats still 2.5 years support left and google may support it further considering the device is still in production, not sure. If you don't want that your only option is to get an ipad.
After the eol you may install lineageos, its not recommended because they keep the bootloader unlocked and you will not get the full security updates after eol or there will be some other tablet in the future that supports GrapheneOS, who knows!