Protecting against bitcoin thieves as a casual user
What other measures can a new, non-power user take in order to protect from this type of significant theft, within reason?

For money stored long term, move the coins to a hardware wallet or some form of offline wallet (cold storage).
Use a transaction (rather than importing keys or using an existing seed-phrase) to move the money to a wallet with fresh keys that have never been on any network-connected device.
Theft is not the only risk. Make backups of the wallet and/or seed phrase or keys. Test recovery from those backups. Educate heirs about recovery.
See What kinds of wallets are there?