Self custody is not a problem, what happened is that generating your seed with a ColdCard MK3 would not have enough entropy, making your private key easy to be guessed, thats it. Your MK3 is still usable, but you should NOT use it to generate a seed, instead you have to use any other solution to generate your seed and then load it on your MK3.
[https://iancoleman.io/bip39/](https://iancoleman.io/bip39/)
[https://sparrowwallet.com/](https://sparrowwallet.com/)
[https://electrum.org/](https://electrum.org/)
Your MK3 is not useless, it still works as intended.