Quote from: NeuroticFish on Today at 12:11:23 PM

Heh, some 15 years ago I was maintaining/modifying some old code for Windows CE PDAs. I remember how truncated the API was.

For OP use case I think that C++ is not a bad option, but it would still require quite an amount of work and I am not convinced that so many PDA users would transform their PDAs into cold storage.
I mean if one has a lot of free time for hobby programming, yes, it can be a nice project, especially if he has a PDA for testing.

I think that since the PDAs (Personal Digital Assistants) aren't dedicated to cryptocurrencies but rather using them as a pocket computer, it would be therefore more secure. I don't really understand security through, so correct me if I'm wrong.

TL;DR is above

First, the device is more user-friendly than a ColdCard. That's for sure as touchscreen is infinitely easier to use compared to clunky UI straight out of old dumbphones.
Second, you can use a more complicated password on a PDA compared to dedicated hardware wallet.
Third, there is extremely low risk (but not zero) that you will accidentally leak your private keys, as the device doesn't have any out-of-sight communication methods such as Wi-Fi, Bluetooth, etc. Of course, there remains IrDA, but it does require direct sight and it can be taped over or removed if you are paranoid.
Fourth, it can be truly air-gapped, as after installing a wallet program on the PDA it doesn't require to be connected to the computer anymore. It can transmit relevant information to your phone via generating QR codes. If you accidentally use IrDA or connect it wired to the computer, it should automatically wipe data in order to protect it from leakage. Of course the feature should be optional (for example to update the app), but should require password and display a very explicit warning to turn it off (preferably with dialog to ask user to check if they have a backup and wipe data for uber-paranoid, again optional).
Fifth, many (but not all) of the security features of a ColdCard (such as duress PIN, BIP39 passphrases/25th word, Brick me PIN (that 'kills' the RAM and ROM if entered), dice rolls for provable Bitcoin seed generation, etc.) can be implemented in the app.

Quote from: NeuroticFish on Today at 12:11:23 PM

But for most users I think that's easier, faster and possibly better too to simply buy a HW.

An old PDA lying somewhere is much less suspicious than a hardware wallet. If you happen to be in danger, with the old Palm models that store their data in RAM and have removable batteries you can just pull out them knowing that your Bitcoin/cryptocurrency savings are safe. There is a problem that you might lock out of your Bitcoins whether either when battery goes zero accidentally or deliberately, so it shall support a paper and pen based backup (BIP39).