But remote attestation doesn't completely prevent you from running custom ROMs. Rather the problem is if you want to use various proprietary crapps/websites that insist on performing attestation. So there doesn't seem to be a point into choosing a "dumb phone" besides as some sort of protest. Rather for each of your devices you'll choose having a libre user-representing (ie secure) OS, or a locked down web TV equivalent. I'd personally choose my pocket device to be running secure libre software, and then use any proprietary crapps on a tablet that stays at home. If remote attestation gets so bad that I can't even do things like access store apps on a libre/secure device, then I'll carry two, with the libre/secure device supplying wifi AP to the other.
(this comment is most certainly not meant as any sort of defense of remote attestation)