What I like to do is removing permissions from apps.
You can do so by going to “Settings” > “Apps” and select the app you would like to limit permissions. Usually I remove network access. But if I give network access, then I remove files access “Access files to your home folder”. Any app can still have their own folders for files located in “/home/{user}/snap/{app}/common”). But if I give files access, then I remove “network” access. I don’t give them both (files and network) at the same time.
Personally I would check if firewall is enabled. Write “sudo ufw status” in the terminal, and if it is disabled, then write “sudo ufw enable”.
Also go to “Settings” > “Security & privacy” to review various choices and status of security features.
I would also enable disk encryption, but this is done during installation I think.