I don't see the necessity to use a Linux distro heavyweight like Ubuntu for cold storage. Better suggestions have been given here already. Remember that cold storage of a wallet containing private keys means it is offline and stays offline basically forever.

You don't need fancy stuff installed. You need the bare minimum to operate und use the wallet of your choice to sign PSBTs transfered from your watch-only wallet. Only the watch-only wallet on your mobile or other online device(s) communicates and touches the evil internet.

Proper, safe and redundant backup of your mnemonic recovery words and what other details about your wallet are helpful for any later recovery is mandatory. Personally, I would also setup a fully encrypted filesystem for the cold storage, so that a possible theft of the cold storage device doesn't compromise your private keys. Of course, don't ever loose the filesystem's encryption passphrase. Redundant and safe backup for it is key, too!