On pixel 9 (and maybe earlier) grapheneos devices, you can just disable the USB c port which is more secure than apple’s pairlocking. Combine that with a self destruct pin/password, your data is bulletproof.

Re: whatsapp, I’m pretty sure the android and iphone app use the same backup format. You can manually extract them by doing an unencrypted iPhone backup to your PC and extract the data from that. When I tested an iPhone a few years ago, I was able to move my whatsapp data after the initial setup with that. I’m pretty sure there are commercial tools that do the same thing too.