You can use Electrum wallet instead of custodial ones, it is a recommended BTC wallet that is open source and safe to use, just download Electrum and verify it to be sure everything is safe and secure.

Electrum will give you your seed phrase as you set it up, make sure you write it down and have more than one backup in two different locations. If you do everything correctly in a safe enviroment, maybe you don't even need a flash drive anymore, you can even delete the software from your device as long as you have your seed phrase backup and you repeat this process again to import your funds with your seed phrase whenever you want to spend, but only do this if you are holding a large amount of BTC and for a very long time. But if you want to be viewing your wallet balance regularly then you have to set up Electrum in an air-gapped device and use another one for watching-only, but this process may be difficult for a newbie.