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I would either buy an iPhone or a cheap Android phone (like an Oppo or Huawei) and flash it with CalyxOS [1].

Huawei/Honor phones now have a locked bootloader so unless there is an exploit it's not possible to install anything else than the stock firmware https://www.xda-developers.com/huawei-stop-providing-bootloa...

Yeah, Oppo and Huawei phones are great fit for CalyxOS. Have you seen the list of supported devices for Calyx? It's basically Pixel-only :).