If your example is OK with a dice roll to generate a random mnemonic, i.e. it is uniformly random enough for your scenario, then you can do the same to generate

If your example is OK with a dice roll to generate a random mnemonic, i.e. it is uniformly random enough for your scenario, then you can do the same to generate random parameter k so that the wallet is not doing it for you. You can also code your own wallet like I mentioned before if you do not trust a hardware wallet manufacturer, but somewhere along the line you will probably need to trust something (like trusting the room you are doing this in is not bugged).