What are you trying to secure it against? This is always the first step to think about, also known as "threat assessment" or similar.

Are you trying to secure it against someone transferring the domains out of your account? Make sure you have all the account security up to date and correct (2FA on the account + the email account that has the ownership), and that all the "transfer locks" are setup (EPP codes and so on)

Are you trying to secure your DNS setup against various DNS attacks? Read up on DNS and various security patterns you can implement like DNSSEC and so on.

You're trying to prevent issues regarding DNS uptime (in case the main domain stops resolving, or other system problems)? In that case, setup DNS over multiple TLDs (and registrars) and have a process for failing over.

In short, it depends on what you're trying to protect against, and who you are trying to protect it against.