Is there a way to add hardware into a phone that is separate from the phone itself but can still interact with it from the inside to sign transactions and truly keep your bitcoins in your phone?
Theoretically it's possible. It should be similar with dual system computer case where you use WiFi/LAN/USB to make connection between system. But i expect it would increase engineering complexity and there's no market for it.
Has anyone seen the Solana Saga phone?
I saw the news, but wouldn't trust them knowing knowing Solana controversy (such as hidden circulating supply/hidden wallet and the network going down frequently).
I've read into the Saga phone the other post mentioned, and it does seem to service the market of people who want all four of security reliability convenience and mobility when it comes to
hotwallets.
They have this thing called a "seed vault"
https://github.com/solana-mobile/seed-vault-sdkIt's part of an open source software collection called the SMS or Solana Mobile Stack.
https://github.com/solana-mobile/solana-mobile-stack-sdkAnyone here that's good at reading code have any input on if something like this is good or not?
Not an answer, but to review hardware wallet, you should review both software and hardware to determine the security. Besides, there are some security expert who would try to crack hardware wallet. Don't forget few expert quickly debunk security of BitFi security (hardware wallet promoted by mcafee) in the past.